diff --git a/idl/SyntaxStateMachine.json5 b/idl/SyntaxStateMachine.json5
index d47656b7..c2d8aff1 100644
--- a/idl/SyntaxStateMachine.json5
+++ b/idl/SyntaxStateMachine.json5
@@ -29,7 +29,7 @@
         { name: "TypeDescr", doc: [ "reflected c++ type" ], definition: "xo::reflect::TypeDescr" },
         { name: "AGCObjectVisitor", doc: [ "gc visitor interface" ], definition: "xo::mm::AGCObjectVisitor" },
         { name: "AGCObject", doc: [ "gc-aware object" ], definition: "xo::mm::AGCObject" },
-        // { name: string, doc: [ string ], definition: string },
+        { name: "VisitReason", doc: [ "hint when traversing gco graph" ], definition: "xo::mm::VisitReason" },
     ],
     const_methods: [
         {
@@ -151,6 +151,7 @@
             doc: ["gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each"],
             return_type: "void",
             args: [
+                {type: "VisitReason", name: "reason"},
                 {type: "obj<AGCObjectVisitor>", name: "gc"},
             ],
         }
diff --git a/include/xo/reader2/DDefineSsm.hpp b/include/xo/reader2/DDefineSsm.hpp
index e31c3b2a..36afb4b4 100644
--- a/include/xo/reader2/DDefineSsm.hpp
+++ b/include/xo/reader2/DDefineSsm.hpp
@@ -74,6 +74,7 @@ namespace xo {
             using Super = DSyntaxStateMachine<DDefineSsm>;
             using TypeDescr = xo::reflect::TypeDescr;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using DArena = xo::mm::DArena;
             using ppindentinfo = xo::print::ppindentinfo;
@@ -200,7 +201,8 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/DExpectExprSsm.hpp b/include/xo/reader2/DExpectExprSsm.hpp
index e73e1f16..b0d4b5f3 100644
--- a/include/xo/reader2/DExpectExprSsm.hpp
+++ b/include/xo/reader2/DExpectExprSsm.hpp
@@ -18,6 +18,7 @@ namespace xo {
             using Super = DSyntaxStateMachine<DExpectExprSsm>;
             using TypeDescr = xo::reflect::TypeDescr;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using DArena = xo::mm::DArena;
             using ppindentinfo = xo::print::ppindentinfo;
 
@@ -194,7 +195,7 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason, obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/DExpectFormalArglistSsm.hpp b/include/xo/reader2/DExpectFormalArglistSsm.hpp
index 954cdc72..c321b135 100644
--- a/include/xo/reader2/DExpectFormalArglistSsm.hpp
+++ b/include/xo/reader2/DExpectFormalArglistSsm.hpp
@@ -56,6 +56,7 @@ namespace xo {
         public:
             using Super = DSyntaxStateMachine<DExpectFormalArglistSsm>;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using DArena = xo::mm::DArena;
             using TypeDescr = xo::reflect::TypeDescr;
@@ -130,7 +131,8 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/DExpectQArraySsm.hpp b/include/xo/reader2/DExpectQArraySsm.hpp
index 2ec32eb4..fcd1a14e 100644
--- a/include/xo/reader2/DExpectQArraySsm.hpp
+++ b/include/xo/reader2/DExpectQArraySsm.hpp
@@ -58,6 +58,7 @@ namespace xo {
         public:
             using Super = DSyntaxStateMachine<DExpectQArraySsm>;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using DArena = xo::mm::DArena;
             using TypeDescr = xo::reflect::TypeDescr;
@@ -123,7 +124,8 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/DExpectQListSsm.hpp b/include/xo/reader2/DExpectQListSsm.hpp
index 3b9ab53d..8c9185ca 100644
--- a/include/xo/reader2/DExpectQListSsm.hpp
+++ b/include/xo/reader2/DExpectQListSsm.hpp
@@ -59,6 +59,7 @@ namespace xo {
         public:
             using Super = DSyntaxStateMachine<DExpectQListSsm>;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using DArena = xo::mm::DArena;
             using TypeDescr = xo::reflect::TypeDescr;
@@ -124,7 +125,7 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason, obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/DExpectQLiteralSsm.hpp b/include/xo/reader2/DExpectQLiteralSsm.hpp
index 2538fb6f..58d7a0b4 100644
--- a/include/xo/reader2/DExpectQLiteralSsm.hpp
+++ b/include/xo/reader2/DExpectQLiteralSsm.hpp
@@ -16,6 +16,7 @@ namespace xo {
         public:
             using Super = DSyntaxStateMachine<DExpectQLiteralSsm>;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using DArena = xo::mm::DArena;
             using TypeDescr = xo::reflect::TypeDescr;
@@ -129,7 +130,7 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason, obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/DExpectSymbolSsm.hpp b/include/xo/reader2/DExpectSymbolSsm.hpp
index f4c3c156..0acd75c3 100644
--- a/include/xo/reader2/DExpectSymbolSsm.hpp
+++ b/include/xo/reader2/DExpectSymbolSsm.hpp
@@ -22,6 +22,7 @@ namespace xo {
         public:
             using Super = DSyntaxStateMachine<DExpectSymbolSsm>;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using DArena = xo::mm::DArena;
             using TypeDescr = xo::reflect::TypeDescr;
             using ppindentinfo = xo::print::ppindentinfo;
@@ -80,7 +81,7 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason, obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
         };
diff --git a/include/xo/reader2/DExpectTypeSsm.hpp b/include/xo/reader2/DExpectTypeSsm.hpp
index d405e916..fd65a3ec 100644
--- a/include/xo/reader2/DExpectTypeSsm.hpp
+++ b/include/xo/reader2/DExpectTypeSsm.hpp
@@ -29,6 +29,7 @@ namespace xo {
             using Super = DSyntaxStateMachine<DExpectTypeSsm>;
             using TypeDescr = xo::reflect::TypeDescr;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using DArena = xo::mm::DArena;
             using ppindentinfo = xo::print::ppindentinfo;
 
@@ -87,7 +88,8 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/DGlobalEnv.hpp b/include/xo/reader2/DGlobalEnv.hpp
index 6bb88f49..02a4679a 100644
--- a/include/xo/reader2/DGlobalEnv.hpp
+++ b/include/xo/reader2/DGlobalEnv.hpp
@@ -27,9 +27,9 @@ namespace xo {
         class DGlobalEnv {
         public:
             using TypeDescr = xo::reflect::TypeDescr;
-            //using ACollector = xo::mm::ACollector;
             using AGCObject = xo::mm::AGCObject;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using MemorySizeVisitor = xo::mm::MemorySizeVisitor;
             using ppindentinfo = xo::print::ppindentinfo;
@@ -73,7 +73,7 @@ namespace xo {
             ///@{
 
             DGlobalEnv * gco_shallow_move(obj<AGCObjectVisitor> gc) noexcept;
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason, obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
             /** @defgroup scm-globalenv-printable-facet **/
diff --git a/include/xo/reader2/DProgressSsm.hpp b/include/xo/reader2/DProgressSsm.hpp
index 372b3af6..3abc38ea 100644
--- a/include/xo/reader2/DProgressSsm.hpp
+++ b/include/xo/reader2/DProgressSsm.hpp
@@ -90,6 +90,7 @@ namespace xo {
             using Super = DSyntaxStateMachine<DProgressSsm>;
             using TypeDescr = xo::reflect::TypeDescr;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using DArena = xo::mm::DArena;
             using ppindentinfo = xo::print::ppindentinfo;
 
@@ -217,7 +218,8 @@ namespace xo {
             /** @defgroup scm-progressssm-gc-support gc support methods **/
             ///@{
 
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/DQuoteSsm.hpp b/include/xo/reader2/DQuoteSsm.hpp
index 542342ef..0874b549 100644
--- a/include/xo/reader2/DQuoteSsm.hpp
+++ b/include/xo/reader2/DQuoteSsm.hpp
@@ -66,6 +66,7 @@ namespace xo {
             using Super = DSyntaxStateMachine<DQuoteSsm>;
             using TypeDescr = xo::reflect::TypeDescr;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using DArena = xo::mm::DArena;
             using ppindentinfo = xo::print::ppindentinfo;
@@ -150,7 +151,7 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason, obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
         private:
diff --git a/include/xo/reader2/DSequenceSsm.hpp b/include/xo/reader2/DSequenceSsm.hpp
index a7819048..c153d76e 100644
--- a/include/xo/reader2/DSequenceSsm.hpp
+++ b/include/xo/reader2/DSequenceSsm.hpp
@@ -30,6 +30,7 @@ namespace xo {
         public:
             using Super = DSyntaxStateMachine<DSequenceSsm>;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using DArena = xo::mm::DArena;
             using ppindentinfo = xo::print::ppindentinfo;
@@ -97,7 +98,8 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/DToplevelSeqSsm.hpp b/include/xo/reader2/DToplevelSeqSsm.hpp
index 4103f45a..5df90f4d 100644
--- a/include/xo/reader2/DToplevelSeqSsm.hpp
+++ b/include/xo/reader2/DToplevelSeqSsm.hpp
@@ -41,6 +41,7 @@ namespace xo {
             using Super = DSyntaxStateMachine<DToplevelSeqSsm>;
             using TypeDescr = xo::reflect::TypeDescr;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using DArena = xo::mm::DArena;
             using ppindentinfo = xo::print::ppindentinfo;
@@ -161,7 +162,7 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason, obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/ParserResult.hpp b/include/xo/reader2/ParserResult.hpp
index 823372da..c8976da4 100644
--- a/include/xo/reader2/ParserResult.hpp
+++ b/include/xo/reader2/ParserResult.hpp
@@ -33,6 +33,7 @@ namespace xo {
         class ParserResult {
         public:
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using ppindentinfo = xo::print::ppindentinfo;
 
         public:
@@ -69,7 +70,7 @@ namespace xo {
             bool pretty(const ppindentinfo & ppii) const;
 
             /** gc support: forward gc-eligible children **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason, obj<AGCObjectVisitor> gc) noexcept;
 
         public:
             /** none|expression|error_description
diff --git a/include/xo/reader2/ParserStack.hpp b/include/xo/reader2/ParserStack.hpp
index e4b414b4..fade2079 100644
--- a/include/xo/reader2/ParserStack.hpp
+++ b/include/xo/reader2/ParserStack.hpp
@@ -24,6 +24,7 @@ namespace xo {
         public:
             //using ACollector = xo::mm::ACollector;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using DArena = xo::mm::DArena;
             using ppindentinfo = xo::print::ppindentinfo;
 
@@ -56,7 +57,8 @@ namespace xo {
             /** pretty-printer support **/
             bool pretty(const ppindentinfo & ppii) const;
 
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
         private:
             /** stack pointer: top of stack just before this instance created **/
diff --git a/include/xo/reader2/ParserStateMachine.hpp b/include/xo/reader2/ParserStateMachine.hpp
index fedd574d..b73eac0b 100644
--- a/include/xo/reader2/ParserStateMachine.hpp
+++ b/include/xo/reader2/ParserStateMachine.hpp
@@ -40,10 +40,11 @@ namespace xo {
         public:
             using TypeDescr = xo::reflect::TypeDescr;
             using ACollector = xo::mm::ACollector;
+            using AGCObject = xo::mm::AGCObject;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using ArenaConfig = xo::mm::ArenaConfig;
-            using AGCObject = xo::mm::AGCObject;
             using DArena = xo::mm::DArena;
             using MemorySizeVisitor = xo::mm::MemorySizeVisitor;
             using ArenaHashMapConfig = xo::map::ArenaHashMapConfig;
@@ -354,7 +355,8 @@ namespace xo {
             ///@{
 
             /** update gc-aware exit pointers from this ParserStateMachine **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/SchematikaReader.hpp b/include/xo/reader2/SchematikaReader.hpp
index d70c2c04..fd2f5cc9 100644
--- a/include/xo/reader2/SchematikaReader.hpp
+++ b/include/xo/reader2/SchematikaReader.hpp
@@ -15,12 +15,14 @@ namespace xo {
         struct ReaderResult {
             using ACollector = xo::mm::ACollector;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using span_type = xo::mm::span<const char>;
 
             bool is_tk_error() const { return tk_error_.is_error(); }
 
             /** forward gc-aware pointers (called during gc cycle) **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
             /** schematika expression parsed from input **/
             obj<AExpression> expr_;
@@ -43,6 +45,7 @@ namespace xo {
         public:
             using ACollector = xo::mm::ACollector;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using MemorySizeVisitor = xo::mm::MemorySizeVisitor;
             using span_type = xo::mm::span<const char>;
@@ -110,7 +113,8 @@ namespace xo {
             void reset_to_idle_toplevel();
 
             /** update gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
         private:
             /** tokenizer converts a stream of chars
diff --git a/include/xo/reader2/apply/DApplySsm.hpp b/include/xo/reader2/apply/DApplySsm.hpp
index f7ee1cce..dd724066 100644
--- a/include/xo/reader2/apply/DApplySsm.hpp
+++ b/include/xo/reader2/apply/DApplySsm.hpp
@@ -8,7 +8,6 @@
 #include "DSyntaxStateMachine.hpp"
 #include "syntaxstatetype.hpp"
 #include <xo/expression2/ApplyExpr.hpp>
-//#include <xo/facet/obj.hpp>
 #include <string_view>
 
 
@@ -66,12 +65,11 @@ namespace xo {
             using Super = DSyntaxStateMachine<DApplySsm>;
             using TypeDescr = xo::reflect::TypeDescr;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using DArena = xo::mm::DArena;
             using ppindentinfo = xo::print::ppindentinfo;
 
-            //using Apply = xo::scm::Apply;
-
         public:
             /** @defgroup scm-applyssm-ctors constructors **/
             ///@{
@@ -190,7 +188,8 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/apply/ISyntaxStateMachine_DApplySsm.hpp b/include/xo/reader2/apply/ISyntaxStateMachine_DApplySsm.hpp
index db3a147d..a50740f5 100644
--- a/include/xo/reader2/apply/ISyntaxStateMachine_DApplySsm.hpp
+++ b/include/xo/reader2/apply/ISyntaxStateMachine_DApplySsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DApplySsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DApplySsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DApplySsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/define/ISyntaxStateMachine_DDefineSsm.hpp b/include/xo/reader2/define/ISyntaxStateMachine_DDefineSsm.hpp
index d5338cd5..35df5ded 100644
--- a/include/xo/reader2/define/ISyntaxStateMachine_DDefineSsm.hpp
+++ b/include/xo/reader2/define/ISyntaxStateMachine_DDefineSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DDefineSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DDefineSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DDefineSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/deftype/DDeftypeSsm.hpp b/include/xo/reader2/deftype/DDeftypeSsm.hpp
index b1a6d910..8bbe3ea1 100644
--- a/include/xo/reader2/deftype/DDeftypeSsm.hpp
+++ b/include/xo/reader2/deftype/DDeftypeSsm.hpp
@@ -67,6 +67,7 @@ namespace xo {
             using Super = DSyntaxStateMachine<DDeftypeSsm>;
             using TypeDescr = xo::reflect::TypeDescr;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using DArena = xo::mm::DArena;
             using ppindentinfo = xo::print::ppindentinfo;
@@ -172,7 +173,7 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason, obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/deftype/ISyntaxStateMachine_DDeftypeSsm.hpp b/include/xo/reader2/deftype/ISyntaxStateMachine_DDeftypeSsm.hpp
index 9bfdfba5..b984bc7d 100644
--- a/include/xo/reader2/deftype/ISyntaxStateMachine_DDeftypeSsm.hpp
+++ b/include/xo/reader2/deftype/ISyntaxStateMachine_DDeftypeSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DDeftypeSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DDeftypeSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DDeftypeSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/env/IGCObject_DGlobalEnv.hpp b/include/xo/reader2/env/IGCObject_DGlobalEnv.hpp
index f96deebc..8efa3c27 100644
--- a/include/xo/reader2/env/IGCObject_DGlobalEnv.hpp
+++ b/include/xo/reader2/env/IGCObject_DGlobalEnv.hpp
@@ -45,6 +45,7 @@ namespace xo {
             using AAllocator = xo::mm::AGCObject::AAllocator;
             using ACollector = xo::mm::AGCObject::ACollector;
             using AGCObjectVisitor = xo::mm::AGCObject::AGCObjectVisitor;
+            using VisitReason = xo::mm::AGCObject::VisitReason;
             using Copaque = xo::mm::AGCObject::Copaque;
             using Opaque = xo::mm::AGCObject::Opaque;
             ///@}
@@ -59,7 +60,7 @@ Arguably abusing the word 'visitor' here **/
             /** Invoke fn.visit_child(iface,data) for each child GCObject pointer.
 Context: provides address of data pointer so it can be updated in place
 when @p fn invokes garbage collector reentry point **/
-            static void visit_gco_children(DGlobalEnv & self, obj<AGCObjectVisitor> fn) noexcept;
+            static void visit_gco_children(DGlobalEnv & self, VisitReason reason, obj<AGCObjectVisitor> fn) noexcept;
             ///@}
         };
 
diff --git a/include/xo/reader2/expect_formal_arg/DExpectFormalArgSsm.hpp b/include/xo/reader2/expect_formal_arg/DExpectFormalArgSsm.hpp
index 5c7e6dec..38a4bb3e 100644
--- a/include/xo/reader2/expect_formal_arg/DExpectFormalArgSsm.hpp
+++ b/include/xo/reader2/expect_formal_arg/DExpectFormalArgSsm.hpp
@@ -49,6 +49,7 @@ namespace xo {
             using Super = DSyntaxStateMachine<DExpectFormalArgSsm>;
             using TypeDescr = xo::reflect::TypeDescr;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using DArena = xo::mm::DArena;
             using ppindentinfo = xo::print::ppindentinfo;
 
@@ -126,7 +127,8 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/expect_formal_arg/ISyntaxStateMachine_DExpectFormalArgSsm.hpp b/include/xo/reader2/expect_formal_arg/ISyntaxStateMachine_DExpectFormalArgSsm.hpp
index a880fad3..ccdfc0bd 100644
--- a/include/xo/reader2/expect_formal_arg/ISyntaxStateMachine_DExpectFormalArgSsm.hpp
+++ b/include/xo/reader2/expect_formal_arg/ISyntaxStateMachine_DExpectFormalArgSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DExpectFormalArgSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DExpectFormalArgSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DExpectFormalArgSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/expect_listtype/DExpectListTypeSsm.hpp b/include/xo/reader2/expect_listtype/DExpectListTypeSsm.hpp
index eeadd15c..ce69f929 100644
--- a/include/xo/reader2/expect_listtype/DExpectListTypeSsm.hpp
+++ b/include/xo/reader2/expect_listtype/DExpectListTypeSsm.hpp
@@ -63,6 +63,7 @@ namespace xo {
             using Super = DSyntaxStateMachine<DExpectListTypeSsm>;
             using TypeDescr = xo::reflect::TypeDescr;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using DArena = xo::mm::DArena;
             using ppindentinfo = xo::print::ppindentinfo;
 
@@ -135,7 +136,8 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
         private:
diff --git a/include/xo/reader2/expect_listtype/ISyntaxStateMachine_DExpectListTypeSsm.hpp b/include/xo/reader2/expect_listtype/ISyntaxStateMachine_DExpectListTypeSsm.hpp
index 9ed4214c..b320b9c9 100644
--- a/include/xo/reader2/expect_listtype/ISyntaxStateMachine_DExpectListTypeSsm.hpp
+++ b/include/xo/reader2/expect_listtype/ISyntaxStateMachine_DExpectListTypeSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DExpectListTypeSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DExpectListTypeSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DExpectListTypeSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/expect_qdict/DExpectQDictSsm.hpp b/include/xo/reader2/expect_qdict/DExpectQDictSsm.hpp
index 5ed40fab..18c52bac 100644
--- a/include/xo/reader2/expect_qdict/DExpectQDictSsm.hpp
+++ b/include/xo/reader2/expect_qdict/DExpectQDictSsm.hpp
@@ -64,6 +64,7 @@ namespace xo {
         public:
             using Super = DSyntaxStateMachine<DExpectQDictSsm>;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using DArena = xo::mm::DArena;
             using ppindentinfo = xo::print::ppindentinfo;
 
@@ -159,7 +160,8 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/expect_qdict/ISyntaxStateMachine_DExpectQDictSsm.hpp b/include/xo/reader2/expect_qdict/ISyntaxStateMachine_DExpectQDictSsm.hpp
index c629e07b..006e43b1 100644
--- a/include/xo/reader2/expect_qdict/ISyntaxStateMachine_DExpectQDictSsm.hpp
+++ b/include/xo/reader2/expect_qdict/ISyntaxStateMachine_DExpectQDictSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DExpectQDictSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DExpectQDictSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DExpectQDictSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/ifelse/DIfElseSsm.hpp b/include/xo/reader2/ifelse/DIfElseSsm.hpp
index fd7b8168..3b63cdb7 100644
--- a/include/xo/reader2/ifelse/DIfElseSsm.hpp
+++ b/include/xo/reader2/ifelse/DIfElseSsm.hpp
@@ -56,6 +56,7 @@ namespace xo {
         public:
             using Super = DSyntaxStateMachine<DIfElseSsm>;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using DArena = xo::mm::DArena;
             using TypeDescr = xo::reflect::TypeDescr;
@@ -171,7 +172,8 @@ namespace xo {
             /** @defgroup scm-ifelsessm-gc-support gc support methods **/
             ///@{
 
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/ifelse/ISyntaxStateMachine_DIfElseSsm.hpp b/include/xo/reader2/ifelse/ISyntaxStateMachine_DIfElseSsm.hpp
index 17f6b4cd..a88e9af1 100644
--- a/include/xo/reader2/ifelse/ISyntaxStateMachine_DIfElseSsm.hpp
+++ b/include/xo/reader2/ifelse/ISyntaxStateMachine_DIfElseSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DIfElseSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DIfElseSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DIfElseSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/lambda/DLambdaSsm.hpp b/include/xo/reader2/lambda/DLambdaSsm.hpp
index 7b16503b..93642f5f 100644
--- a/include/xo/reader2/lambda/DLambdaSsm.hpp
+++ b/include/xo/reader2/lambda/DLambdaSsm.hpp
@@ -60,6 +60,7 @@ namespace xo {
             using Super = DSyntaxStateMachine<DLambdaSsm>;
             using DLocalSymtab = xo::scm::DLocalSymtab;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using DArena = xo::mm::DArena;
             using TypeDescr = xo::reflect::TypeDescr;
@@ -189,7 +190,8 @@ namespace xo {
             ///@{
 
             /** gc support: visit gc-aware child pointers **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/lambda/ISyntaxStateMachine_DLambdaSsm.hpp b/include/xo/reader2/lambda/ISyntaxStateMachine_DLambdaSsm.hpp
index db67ee9a..baeb5af4 100644
--- a/include/xo/reader2/lambda/ISyntaxStateMachine_DLambdaSsm.hpp
+++ b/include/xo/reader2/lambda/ISyntaxStateMachine_DLambdaSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DLambdaSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DLambdaSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DLambdaSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/paren/DParenSsm.hpp b/include/xo/reader2/paren/DParenSsm.hpp
index e70ba255..9da3ba42 100644
--- a/include/xo/reader2/paren/DParenSsm.hpp
+++ b/include/xo/reader2/paren/DParenSsm.hpp
@@ -51,6 +51,7 @@ namespace xo {
             using Super = DSyntaxStateMachine<DParenSsm>;
             using TypeDescr = xo::reflect::TypeDescr;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using DArena = xo::mm::DArena;
             using ppindentinfo = xo::print::ppindentinfo;
@@ -136,7 +137,8 @@ namespace xo {
             ///@{
 
             /** gc support: visit immediate gc-aware children **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason,
+                                    obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/reader2/paren/ISyntaxStateMachine_DParenSsm.hpp b/include/xo/reader2/paren/ISyntaxStateMachine_DParenSsm.hpp
index 4dd7ef74..dc1a826d 100644
--- a/include/xo/reader2/paren/ISyntaxStateMachine_DParenSsm.hpp
+++ b/include/xo/reader2/paren/ISyntaxStateMachine_DParenSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DParenSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DParenSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DParenSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/parser/DSchematikaParser.hpp b/include/xo/reader2/parser/DSchematikaParser.hpp
index 24e4b46a..f699c30b 100644
--- a/include/xo/reader2/parser/DSchematikaParser.hpp
+++ b/include/xo/reader2/parser/DSchematikaParser.hpp
@@ -164,9 +164,9 @@ namespace xo {
             using token_type = Token;
             using ArenaHashMapConfig = xo::map::ArenaHashMapConfig;
             using ArenaConfig = xo::mm::ArenaConfig;
-            //using ACollector = xo::mm::ACollector;
             using AGCObject = xo::mm::AGCObject;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using VisitReason = xo::mm::VisitReason;
             using AAllocator = xo::mm::AAllocator;
             using MemorySizeVisitor = xo::mm::MemorySizeVisitor;
             using ppindentinfo = xo::print::ppindentinfo;
@@ -297,7 +297,7 @@ namespace xo {
             /** not implemented (SchematikaParser not designed to be copyable) **/
             DSchematikaParser * gco_shallow_move(obj<AGCObjectVisitor> gc) noexcept;
             /** forward gc-aware children **/
-            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
+            void visit_gco_children(VisitReason reason, obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
         private:
diff --git a/include/xo/reader2/parser/IGCObject_DSchematikaParser.hpp b/include/xo/reader2/parser/IGCObject_DSchematikaParser.hpp
index 64394e65..586678cd 100644
--- a/include/xo/reader2/parser/IGCObject_DSchematikaParser.hpp
+++ b/include/xo/reader2/parser/IGCObject_DSchematikaParser.hpp
@@ -45,6 +45,7 @@ namespace xo {
             using AAllocator = xo::mm::AGCObject::AAllocator;
             using ACollector = xo::mm::AGCObject::ACollector;
             using AGCObjectVisitor = xo::mm::AGCObject::AGCObjectVisitor;
+            using VisitReason = xo::mm::AGCObject::VisitReason;
             using Copaque = xo::mm::AGCObject::Copaque;
             using Opaque = xo::mm::AGCObject::Opaque;
             ///@}
@@ -59,7 +60,7 @@ Arguably abusing the word 'visitor' here **/
             /** Invoke fn.visit_child(iface,data) for each child GCObject pointer.
 Context: provides address of data pointer so it can be updated in place
 when @p fn invokes garbage collector reentry point **/
-            static void visit_gco_children(DSchematikaParser & self, obj<AGCObjectVisitor> fn) noexcept;
+            static void visit_gco_children(DSchematikaParser & self, VisitReason reason, obj<AGCObjectVisitor> fn) noexcept;
             ///@}
         };
 
diff --git a/include/xo/reader2/quote/ISyntaxStateMachine_DQuoteSsm.hpp b/include/xo/reader2/quote/ISyntaxStateMachine_DQuoteSsm.hpp
index 35dc5da1..0f01d967 100644
--- a/include/xo/reader2/quote/ISyntaxStateMachine_DQuoteSsm.hpp
+++ b/include/xo/reader2/quote/ISyntaxStateMachine_DQuoteSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DQuoteSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DQuoteSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DQuoteSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/ssm/ASyntaxStateMachine.hpp b/include/xo/reader2/ssm/ASyntaxStateMachine.hpp
index 07d992c1..fc71175b 100644
--- a/include/xo/reader2/ssm/ASyntaxStateMachine.hpp
+++ b/include/xo/reader2/ssm/ASyntaxStateMachine.hpp
@@ -50,6 +50,8 @@ public:
     using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
     /** gc-aware object **/
     using AGCObject = xo::mm::AGCObject;
+    /** hint when traversing gco graph **/
+    using VisitReason = xo::mm::VisitReason;
     ///@}
 
     /** @defgroup scm-syntaxstatemachine-methods **/
@@ -91,7 +93,7 @@ public:
     /** update state machine for nested quoted literal @p lit **/
     virtual void on_quoted_literal(Opaque data, obj<AGCObject> lit, ParserStateMachine * p_psm)  = 0;
     /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-    virtual void visit_gco_children(Opaque data, obj<AGCObjectVisitor> gc)  = 0;
+    virtual void visit_gco_children(Opaque data, VisitReason reason, obj<AGCObjectVisitor> gc)  = 0;
     ///@}
 }; /*ASyntaxStateMachine*/
 
diff --git a/include/xo/reader2/ssm/ISyntaxStateMachine_Any.hpp b/include/xo/reader2/ssm/ISyntaxStateMachine_Any.hpp
index 34e7ec84..88b0cde2 100644
--- a/include/xo/reader2/ssm/ISyntaxStateMachine_Any.hpp
+++ b/include/xo/reader2/ssm/ISyntaxStateMachine_Any.hpp
@@ -47,6 +47,7 @@ namespace scm {
         using TypeDescr = ASyntaxStateMachine::TypeDescr;
         using AGCObjectVisitor = ASyntaxStateMachine::AGCObjectVisitor;
         using AGCObject = ASyntaxStateMachine::AGCObject;
+        using VisitReason = ASyntaxStateMachine::VisitReason;
 
         ///@}
         /** @defgroup scm-syntaxstatemachine-any-methods **/
@@ -75,7 +76,7 @@ namespace scm {
         [[noreturn]] void on_parsed_expression(Opaque, obj<AExpression>, ParserStateMachine *)  override;
         [[noreturn]] void on_parsed_expression_with_token(Opaque, obj<AExpression>, const Token &, ParserStateMachine *)  override;
         [[noreturn]] void on_quoted_literal(Opaque, obj<AGCObject>, ParserStateMachine *)  override;
-        [[noreturn]] void visit_gco_children(Opaque, obj<AGCObjectVisitor>)  override;
+        [[noreturn]] void visit_gco_children(Opaque, VisitReason, obj<AGCObjectVisitor>)  override;
 
         ///@}
 
diff --git a/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectExprSsm.hpp b/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectExprSsm.hpp
index 5ca9f1c0..63210b94 100644
--- a/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectExprSsm.hpp
+++ b/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectExprSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DExpectExprSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DExpectExprSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DExpectExprSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectFormalArglistSsm.hpp b/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectFormalArglistSsm.hpp
index 04da69bd..8b9d4083 100644
--- a/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectFormalArglistSsm.hpp
+++ b/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectFormalArglistSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DExpectFormalArglistSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DExpectFormalArglistSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DExpectFormalArglistSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectQArraySsm.hpp b/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectQArraySsm.hpp
index 7b345008..e4b15c21 100644
--- a/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectQArraySsm.hpp
+++ b/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectQArraySsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DExpectQArraySsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DExpectQArraySsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DExpectQArraySsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectQListSsm.hpp b/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectQListSsm.hpp
index ea0cb3c2..42fdc804 100644
--- a/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectQListSsm.hpp
+++ b/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectQListSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DExpectQListSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DExpectQListSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DExpectQListSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectQLiteralSsm.hpp b/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectQLiteralSsm.hpp
index 4a4bdbfb..5dc2130b 100644
--- a/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectQLiteralSsm.hpp
+++ b/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectQLiteralSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DExpectQLiteralSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DExpectQLiteralSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DExpectQLiteralSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectSymbolSsm.hpp b/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectSymbolSsm.hpp
index e3cf7146..dfc0d6bb 100644
--- a/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectSymbolSsm.hpp
+++ b/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectSymbolSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DExpectSymbolSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DExpectSymbolSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DExpectSymbolSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectTypeSsm.hpp b/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectTypeSsm.hpp
index 965212d1..b8607deb 100644
--- a/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectTypeSsm.hpp
+++ b/include/xo/reader2/ssm/ISyntaxStateMachine_DExpectTypeSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DExpectTypeSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DExpectTypeSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DExpectTypeSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/ssm/ISyntaxStateMachine_DProgressSsm.hpp b/include/xo/reader2/ssm/ISyntaxStateMachine_DProgressSsm.hpp
index 7c12ee39..3a2b3aaf 100644
--- a/include/xo/reader2/ssm/ISyntaxStateMachine_DProgressSsm.hpp
+++ b/include/xo/reader2/ssm/ISyntaxStateMachine_DProgressSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DProgressSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DProgressSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DProgressSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/ssm/ISyntaxStateMachine_DSequenceSsm.hpp b/include/xo/reader2/ssm/ISyntaxStateMachine_DSequenceSsm.hpp
index b35d45a1..74183d99 100644
--- a/include/xo/reader2/ssm/ISyntaxStateMachine_DSequenceSsm.hpp
+++ b/include/xo/reader2/ssm/ISyntaxStateMachine_DSequenceSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DSequenceSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DSequenceSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DSequenceSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/ssm/ISyntaxStateMachine_DToplevelSeqSsm.hpp b/include/xo/reader2/ssm/ISyntaxStateMachine_DToplevelSeqSsm.hpp
index 695da17c..55da530d 100644
--- a/include/xo/reader2/ssm/ISyntaxStateMachine_DToplevelSeqSsm.hpp
+++ b/include/xo/reader2/ssm/ISyntaxStateMachine_DToplevelSeqSsm.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using TypeDescr = xo::scm::ASyntaxStateMachine::TypeDescr;
             using AGCObjectVisitor = xo::scm::ASyntaxStateMachine::AGCObjectVisitor;
             using AGCObject = xo::scm::ASyntaxStateMachine::AGCObject;
+            using VisitReason = xo::scm::ASyntaxStateMachine::VisitReason;
             using Copaque = xo::scm::ASyntaxStateMachine::Copaque;
             using Opaque = xo::scm::ASyntaxStateMachine::Opaque;
             ///@}
@@ -77,7 +78,7 @@ namespace xo {
             /** update state machine for nested quoted literal @p lit **/
             static void on_quoted_literal(DToplevelSeqSsm & self, obj<AGCObject> lit, ParserStateMachine * p_psm);
             /** gc support: visit immediate gc-aware child pointers with @p gc.  Call gc.visit_child() for each **/
-            static void visit_gco_children(DToplevelSeqSsm & self, obj<AGCObjectVisitor> gc);
+            static void visit_gco_children(DToplevelSeqSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc);
             ///@}
         };
 
diff --git a/include/xo/reader2/ssm/ISyntaxStateMachine_Xfer.hpp b/include/xo/reader2/ssm/ISyntaxStateMachine_Xfer.hpp
index df1a87f4..1d971d3b 100644
--- a/include/xo/reader2/ssm/ISyntaxStateMachine_Xfer.hpp
+++ b/include/xo/reader2/ssm/ISyntaxStateMachine_Xfer.hpp
@@ -36,6 +36,7 @@ namespace scm {
         using TypeDescr = ASyntaxStateMachine::TypeDescr;
         using AGCObjectVisitor = ASyntaxStateMachine::AGCObjectVisitor;
         using AGCObject = ASyntaxStateMachine::AGCObject;
+        using VisitReason = ASyntaxStateMachine::VisitReason;
         ///@}
 
         /** @defgroup scm-syntaxstatemachine-xfer-methods **/
@@ -89,8 +90,8 @@ namespace scm {
         void on_quoted_literal(Opaque data, obj<AGCObject> lit, ParserStateMachine * p_psm)  override {
             return I::on_quoted_literal(_dcast(data), lit, p_psm);
         }
-        void visit_gco_children(Opaque data, obj<AGCObjectVisitor> gc)  override {
-            return I::visit_gco_children(_dcast(data), gc);
+        void visit_gco_children(Opaque data, VisitReason reason, obj<AGCObjectVisitor> gc)  override {
+            return I::visit_gco_children(_dcast(data), reason, gc);
         }
 
         ///@}
diff --git a/include/xo/reader2/ssm/RSyntaxStateMachine.hpp b/include/xo/reader2/ssm/RSyntaxStateMachine.hpp
index 9c8a8274..2f5b9fb0 100644
--- a/include/xo/reader2/ssm/RSyntaxStateMachine.hpp
+++ b/include/xo/reader2/ssm/RSyntaxStateMachine.hpp
@@ -34,6 +34,7 @@ public:
     using TypeDescr = ASyntaxStateMachine::TypeDescr;
     using AGCObjectVisitor = ASyntaxStateMachine::AGCObjectVisitor;
     using AGCObject = ASyntaxStateMachine::AGCObject;
+    using VisitReason = ASyntaxStateMachine::VisitReason;
     ///@}
 
     /** @defgroup scm-syntaxstatemachine-router-ctors **/
@@ -93,8 +94,8 @@ public:
     void on_quoted_literal(obj<AGCObject> lit, ParserStateMachine * p_psm)  {
         return O::iface()->on_quoted_literal(O::data(), lit, p_psm);
     }
-    void visit_gco_children(obj<AGCObjectVisitor> gc)  {
-        return O::iface()->visit_gco_children(O::data(), gc);
+    void visit_gco_children(VisitReason reason, obj<AGCObjectVisitor> gc)  {
+        return O::iface()->visit_gco_children(O::data(), reason, gc);
     }
 
     ///@}
diff --git a/src/reader2/DApplySsm.cpp b/src/reader2/DApplySsm.cpp
index 1787d560..ef84187b 100644
--- a/src/reader2/DApplySsm.cpp
+++ b/src/reader2/DApplySsm.cpp
@@ -397,10 +397,11 @@ namespace xo {
         }
 
         void
-        DApplySsm::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        DApplySsm::visit_gco_children(VisitReason reason,
+                                      obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.visit_poly_child(&fn_expr_);
-            gc.visit_child(&args_expr_v_);
+            gc.visit_poly_child(reason, &fn_expr_);
+            gc.visit_child(reason, &args_expr_v_);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/DDefineSsm.cpp b/src/reader2/DDefineSsm.cpp
index 1ee8e3aa..0e38f93d 100644
--- a/src/reader2/DDefineSsm.cpp
+++ b/src/reader2/DDefineSsm.cpp
@@ -692,9 +692,10 @@ namespace xo {
         // ----- gc support -----
 
         void
-        DDefineSsm::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        DDefineSsm::visit_gco_children(VisitReason reason,
+                                       obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.visit_child(&def_expr_.data_);
+            gc.visit_child(reason, &def_expr_.data_);
         }
     } /*namespace scm*/
 } /*namespace xo*/
diff --git a/src/reader2/DDeftypeSsm.cpp b/src/reader2/DDeftypeSsm.cpp
index 0007d940..108f6be7 100644
--- a/src/reader2/DDeftypeSsm.cpp
+++ b/src/reader2/DDeftypeSsm.cpp
@@ -271,7 +271,7 @@ namespace xo {
                                              refrtag("expect", this->get_expect_str()));
         }
         void
-        DDeftypeSsm::visit_gco_children(obj<AGCObjectVisitor>) noexcept
+        DDeftypeSsm::visit_gco_children(VisitReason, obj<AGCObjectVisitor>) noexcept
         {
             static_assert(!DUniqueString::is_gc_eligible());
         }
diff --git a/src/reader2/DExpectExprSsm.cpp b/src/reader2/DExpectExprSsm.cpp
index bce870f6..9526aaea 100644
--- a/src/reader2/DExpectExprSsm.cpp
+++ b/src/reader2/DExpectExprSsm.cpp
@@ -625,7 +625,8 @@ namespace xo {
 #endif
 
         void
-        DExpectExprSsm::visit_gco_children(obj<AGCObjectVisitor>) noexcept
+        DExpectExprSsm::visit_gco_children(VisitReason,
+                                           obj<AGCObjectVisitor>) noexcept
         {
             // all members POD, skip
         }
diff --git a/src/reader2/DExpectFormalArgSsm.cpp b/src/reader2/DExpectFormalArgSsm.cpp
index 7a529983..f1bdd81d 100644
--- a/src/reader2/DExpectFormalArgSsm.cpp
+++ b/src/reader2/DExpectFormalArgSsm.cpp
@@ -265,7 +265,8 @@ namespace xo {
         }
 
         void
-        DExpectFormalArgSsm::visit_gco_children(obj<AGCObjectVisitor>) noexcept
+        DExpectFormalArgSsm::visit_gco_children(VisitReason,
+                                                obj<AGCObjectVisitor>) noexcept
         {
             static_assert(!DUniqueString::is_gc_eligible());
         }
diff --git a/src/reader2/DExpectFormalArglistSsm.cpp b/src/reader2/DExpectFormalArglistSsm.cpp
index 6cbf199a..6d68a610 100644
--- a/src/reader2/DExpectFormalArglistSsm.cpp
+++ b/src/reader2/DExpectFormalArglistSsm.cpp
@@ -358,9 +358,10 @@ namespace xo {
         }
 
         void
-        DExpectFormalArglistSsm::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        DExpectFormalArglistSsm::visit_gco_children(VisitReason reason,
+                                                    obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.visit_child(&argl_);
+            gc.visit_child(reason, &argl_);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/DExpectListTypeSsm.cpp b/src/reader2/DExpectListTypeSsm.cpp
index 001ff254..e94e299b 100644
--- a/src/reader2/DExpectListTypeSsm.cpp
+++ b/src/reader2/DExpectListTypeSsm.cpp
@@ -204,9 +204,10 @@ namespace xo {
         }
 
         void
-        DExpectListTypeSsm::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        DExpectListTypeSsm::visit_gco_children(VisitReason reason,
+                                               obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.visit_poly_child(&elt_type_);
+            gc.visit_poly_child(reason, &elt_type_);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/DExpectQArraySsm.cpp b/src/reader2/DExpectQArraySsm.cpp
index 9e281f8c..97e68228 100644
--- a/src/reader2/DExpectQArraySsm.cpp
+++ b/src/reader2/DExpectQArraySsm.cpp
@@ -220,9 +220,10 @@ namespace xo {
                                              refrtag("array", array_pr));
         }
         void
-        DExpectQArraySsm::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        DExpectQArraySsm::visit_gco_children(VisitReason reason,
+                                             obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.visit_child(&array_);
+            gc.visit_child(reason, &array_);
         }
 
     }
diff --git a/src/reader2/DExpectQDictSsm.cpp b/src/reader2/DExpectQDictSsm.cpp
index 9839ffd2..4a34d19a 100644
--- a/src/reader2/DExpectQDictSsm.cpp
+++ b/src/reader2/DExpectQDictSsm.cpp
@@ -267,10 +267,11 @@ namespace xo {
 
 
         void
-        DExpectQDictSsm::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        DExpectQDictSsm::visit_gco_children(VisitReason reason,
+                                            obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.visit_child(&key_);
-            gc.visit_child(&dict_);
+            gc.visit_child(reason, &key_);
+            gc.visit_child(reason, &dict_);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/DExpectQListSsm.cpp b/src/reader2/DExpectQListSsm.cpp
index 965351c3..5a570caa 100644
--- a/src/reader2/DExpectQListSsm.cpp
+++ b/src/reader2/DExpectQListSsm.cpp
@@ -215,10 +215,11 @@ namespace xo {
                                              refrtag("list", list_pr));
         }
         void
-        DExpectQListSsm::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        DExpectQListSsm::visit_gco_children(VisitReason reason,
+                                            obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.visit_child(&start_);
-            gc.visit_child(&end_);
+            gc.visit_child(reason, &start_);
+            gc.visit_child(reason, &end_);
         }
 
     }
diff --git a/src/reader2/DExpectQLiteralSsm.cpp b/src/reader2/DExpectQLiteralSsm.cpp
index c49cf565..5265aeb7 100644
--- a/src/reader2/DExpectQLiteralSsm.cpp
+++ b/src/reader2/DExpectQLiteralSsm.cpp
@@ -256,7 +256,7 @@ namespace xo {
                                              refrtag("expect", this->get_expect_str()));
         }
         void
-        DExpectQLiteralSsm::visit_gco_children(obj<AGCObjectVisitor>) noexcept
+        DExpectQLiteralSsm::visit_gco_children(VisitReason, obj<AGCObjectVisitor>) noexcept
         {
             // cxl_on_rightparen_, cxl_on_rightbracket_: POD, skip
         }
diff --git a/src/reader2/DExpectSymbolSsm.cpp b/src/reader2/DExpectSymbolSsm.cpp
index fa1372b1..6847a776 100644
--- a/src/reader2/DExpectSymbolSsm.cpp
+++ b/src/reader2/DExpectSymbolSsm.cpp
@@ -146,7 +146,8 @@ namespace xo {
                     );
         }
         void
-        DExpectSymbolSsm::visit_gco_children(obj<AGCObjectVisitor>) noexcept
+        DExpectSymbolSsm::visit_gco_children(VisitReason,
+                                             obj<AGCObjectVisitor>) noexcept
         {
             // no gc-aware members
         }
diff --git a/src/reader2/DExpectTypeSsm.cpp b/src/reader2/DExpectTypeSsm.cpp
index 47cc5a6c..7a5d55a3 100644
--- a/src/reader2/DExpectTypeSsm.cpp
+++ b/src/reader2/DExpectTypeSsm.cpp
@@ -198,7 +198,8 @@ namespace xo {
         }
 
         void
-        DExpectTypeSsm::visit_gco_children(obj<AGCObjectVisitor>) noexcept
+        DExpectTypeSsm::visit_gco_children(VisitReason,
+                                           obj<AGCObjectVisitor>) noexcept
         {
             // corrected_: POD, skip
         }
diff --git a/src/reader2/DGlobalEnv.cpp b/src/reader2/DGlobalEnv.cpp
index 8491c60b..b1eecf37 100644
--- a/src/reader2/DGlobalEnv.cpp
+++ b/src/reader2/DGlobalEnv.cpp
@@ -114,10 +114,11 @@ namespace xo {
         }
 
         void
-        DGlobalEnv::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        DGlobalEnv::visit_gco_children(VisitReason reason,
+                                       obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.visit_child(&symtab_);
-            gc.visit_child(&values_);
+            gc.visit_child(reason, &symtab_);
+            gc.visit_child(reason, &values_);
         }
 
         // ----- APrintable facet -----
diff --git a/src/reader2/DIfElseSsm.cpp b/src/reader2/DIfElseSsm.cpp
index f4fd4f88..5efd9289 100644
--- a/src/reader2/DIfElseSsm.cpp
+++ b/src/reader2/DIfElseSsm.cpp
@@ -510,9 +510,10 @@ namespace xo {
         }
 
         void
-        DIfElseSsm::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        DIfElseSsm::visit_gco_children(VisitReason reason,
+                                       obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.visit_poly_child(&if_expr_);
+            gc.visit_poly_child(reason, &if_expr_);
         }
     } /*namespace scm*/
 } /*namespace xo*/
diff --git a/src/reader2/DLambdaSsm.cpp b/src/reader2/DLambdaSsm.cpp
index f0fed420..e39e530b 100644
--- a/src/reader2/DLambdaSsm.cpp
+++ b/src/reader2/DLambdaSsm.cpp
@@ -471,15 +471,16 @@ namespace xo {
         }
 
         void
-        DLambdaSsm::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        DLambdaSsm::visit_gco_children(VisitReason reason,
+                                       obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.visit_child(&local_symtab_);
+            gc.visit_child(reason, &local_symtab_);
 
             // explicit_return_td not gcobject
             // lambda_td not gcobject
 
-            gc.visit_poly_child(&body_);
-            gc.visit_poly_child(&parent_symtab_);
+            gc.visit_poly_child(reason, &body_);
+            gc.visit_poly_child(reason, &parent_symtab_);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/DParenSsm.cpp b/src/reader2/DParenSsm.cpp
index 82320cb6..a832bd15 100644
--- a/src/reader2/DParenSsm.cpp
+++ b/src/reader2/DParenSsm.cpp
@@ -459,9 +459,10 @@ namespace xo {
         }
 
         void
-        DParenSsm::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        DParenSsm::visit_gco_children(VisitReason reason,
+                                      obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.visit_poly_child(&expr_);
+            gc.visit_poly_child(reason, &expr_);
         }
     } /*namespace scm*/
 } /*namespace xo*/
diff --git a/src/reader2/DProgressSsm.cpp b/src/reader2/DProgressSsm.cpp
index 04c87fac..00ce5f61 100644
--- a/src/reader2/DProgressSsm.cpp
+++ b/src/reader2/DProgressSsm.cpp
@@ -1244,10 +1244,11 @@ case optype::op_assign:
         }
 
         void
-        DProgressSsm::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        DProgressSsm::visit_gco_children(VisitReason reason,
+                                         obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.visit_poly_child(&lhs_);
-            gc.visit_poly_child(&rhs_);
+            gc.visit_poly_child(reason, &lhs_);
+            gc.visit_poly_child(reason, &rhs_);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/DQuoteSsm.cpp b/src/reader2/DQuoteSsm.cpp
index cbbedc73..af5d08e3 100644
--- a/src/reader2/DQuoteSsm.cpp
+++ b/src/reader2/DQuoteSsm.cpp
@@ -212,9 +212,10 @@ namespace xo {
         }
 
         void
-        DQuoteSsm::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        DQuoteSsm::visit_gco_children(VisitReason reason,
+                                      obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.visit_poly_child(&expr_);
+            gc.visit_poly_child(reason, &expr_);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/DSchematikaParser.cpp b/src/reader2/DSchematikaParser.cpp
index fc71d12f..c1a92c5b 100644
--- a/src/reader2/DSchematikaParser.cpp
+++ b/src/reader2/DSchematikaParser.cpp
@@ -197,9 +197,10 @@ namespace xo {
         }
 
         void
-        DSchematikaParser::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        DSchematikaParser::visit_gco_children(VisitReason reason,
+                                              obj<AGCObjectVisitor> gc) noexcept
         {
-            psm_.visit_gco_children(gc);
+            psm_.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/DSequenceSsm.cpp b/src/reader2/DSequenceSsm.cpp
index a6d4013b..c3517065 100644
--- a/src/reader2/DSequenceSsm.cpp
+++ b/src/reader2/DSequenceSsm.cpp
@@ -258,9 +258,10 @@ namespace xo {
         }
 
         void
-        DSequenceSsm::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        DSequenceSsm::visit_gco_children(VisitReason reason,
+                                         obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.visit_child(&seq_expr_);
+            gc.visit_child(reason, &seq_expr_);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/DToplevelSeqSsm.cpp b/src/reader2/DToplevelSeqSsm.cpp
index e660f780..cab947b1 100644
--- a/src/reader2/DToplevelSeqSsm.cpp
+++ b/src/reader2/DToplevelSeqSsm.cpp
@@ -518,7 +518,7 @@ namespace xo {
                  refrtag("seqtype", seqtype_));
         }
         void
-        DToplevelSeqSsm::visit_gco_children(obj<AGCObjectVisitor>) noexcept
+        DToplevelSeqSsm::visit_gco_children(VisitReason, obj<AGCObjectVisitor>) noexcept
         {
             // seqtype_: POD, skip
         }
diff --git a/src/reader2/ISyntaxStateMachine_Any.cpp b/src/reader2/ISyntaxStateMachine_Any.cpp
index a260bdc9..e20b42f6 100644
--- a/src/reader2/ISyntaxStateMachine_Any.cpp
+++ b/src/reader2/ISyntaxStateMachine_Any.cpp
@@ -96,7 +96,7 @@ ISyntaxStateMachine_Any::on_quoted_literal(Opaque, obj<AGCObject>, ParserStateMa
 }
 
 auto
-ISyntaxStateMachine_Any::visit_gco_children(Opaque, obj<AGCObjectVisitor>)  -> void
+ISyntaxStateMachine_Any::visit_gco_children(Opaque, VisitReason, obj<AGCObjectVisitor>)  -> void
 {
     _fatal();
 }
diff --git a/src/reader2/ISyntaxStateMachine_DExpectExprSsm.cpp b/src/reader2/ISyntaxStateMachine_DExpectExprSsm.cpp
index 0ecc4e24..0d6146e4 100644
--- a/src/reader2/ISyntaxStateMachine_DExpectExprSsm.cpp
+++ b/src/reader2/ISyntaxStateMachine_DExpectExprSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DExpectExprSsm::visit_gco_children(DExpectExprSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DExpectExprSsm::visit_gco_children(DExpectExprSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/ISyntaxStateMachine_DExpectFormalArglistSsm.cpp b/src/reader2/ISyntaxStateMachine_DExpectFormalArglistSsm.cpp
index 4db6a480..77bf5f55 100644
--- a/src/reader2/ISyntaxStateMachine_DExpectFormalArglistSsm.cpp
+++ b/src/reader2/ISyntaxStateMachine_DExpectFormalArglistSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DExpectFormalArglistSsm::visit_gco_children(DExpectFormalArglistSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DExpectFormalArglistSsm::visit_gco_children(DExpectFormalArglistSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/ISyntaxStateMachine_DExpectQArraySsm.cpp b/src/reader2/ISyntaxStateMachine_DExpectQArraySsm.cpp
index ea8b14cc..2eb6f6b5 100644
--- a/src/reader2/ISyntaxStateMachine_DExpectQArraySsm.cpp
+++ b/src/reader2/ISyntaxStateMachine_DExpectQArraySsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DExpectQArraySsm::visit_gco_children(DExpectQArraySsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DExpectQArraySsm::visit_gco_children(DExpectQArraySsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/ISyntaxStateMachine_DExpectQListSsm.cpp b/src/reader2/ISyntaxStateMachine_DExpectQListSsm.cpp
index 363f9646..b0f89a2d 100644
--- a/src/reader2/ISyntaxStateMachine_DExpectQListSsm.cpp
+++ b/src/reader2/ISyntaxStateMachine_DExpectQListSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DExpectQListSsm::visit_gco_children(DExpectQListSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DExpectQListSsm::visit_gco_children(DExpectQListSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/ISyntaxStateMachine_DExpectQLiteralSsm.cpp b/src/reader2/ISyntaxStateMachine_DExpectQLiteralSsm.cpp
index 61e15926..69f17c50 100644
--- a/src/reader2/ISyntaxStateMachine_DExpectQLiteralSsm.cpp
+++ b/src/reader2/ISyntaxStateMachine_DExpectQLiteralSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DExpectQLiteralSsm::visit_gco_children(DExpectQLiteralSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DExpectQLiteralSsm::visit_gco_children(DExpectQLiteralSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/ISyntaxStateMachine_DExpectSymbolSsm.cpp b/src/reader2/ISyntaxStateMachine_DExpectSymbolSsm.cpp
index c1331ceb..ea2b7a0d 100644
--- a/src/reader2/ISyntaxStateMachine_DExpectSymbolSsm.cpp
+++ b/src/reader2/ISyntaxStateMachine_DExpectSymbolSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DExpectSymbolSsm::visit_gco_children(DExpectSymbolSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DExpectSymbolSsm::visit_gco_children(DExpectSymbolSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/ISyntaxStateMachine_DExpectTypeSsm.cpp b/src/reader2/ISyntaxStateMachine_DExpectTypeSsm.cpp
index 8daed04c..e107f011 100644
--- a/src/reader2/ISyntaxStateMachine_DExpectTypeSsm.cpp
+++ b/src/reader2/ISyntaxStateMachine_DExpectTypeSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DExpectTypeSsm::visit_gco_children(DExpectTypeSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DExpectTypeSsm::visit_gco_children(DExpectTypeSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/ISyntaxStateMachine_DParenSsm.cpp b/src/reader2/ISyntaxStateMachine_DParenSsm.cpp
index 42a199cb..f67e77c8 100644
--- a/src/reader2/ISyntaxStateMachine_DParenSsm.cpp
+++ b/src/reader2/ISyntaxStateMachine_DParenSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DParenSsm::visit_gco_children(DParenSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DParenSsm::visit_gco_children(DParenSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/ISyntaxStateMachine_DProgressSsm.cpp b/src/reader2/ISyntaxStateMachine_DProgressSsm.cpp
index 9c5ad07d..5ad51fe2 100644
--- a/src/reader2/ISyntaxStateMachine_DProgressSsm.cpp
+++ b/src/reader2/ISyntaxStateMachine_DProgressSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DProgressSsm::visit_gco_children(DProgressSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DProgressSsm::visit_gco_children(DProgressSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/ISyntaxStateMachine_DSequenceSsm.cpp b/src/reader2/ISyntaxStateMachine_DSequenceSsm.cpp
index a6375288..8dff7fa0 100644
--- a/src/reader2/ISyntaxStateMachine_DSequenceSsm.cpp
+++ b/src/reader2/ISyntaxStateMachine_DSequenceSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DSequenceSsm::visit_gco_children(DSequenceSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DSequenceSsm::visit_gco_children(DSequenceSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/ISyntaxStateMachine_DToplevelSeqSsm.cpp b/src/reader2/ISyntaxStateMachine_DToplevelSeqSsm.cpp
index 47788590..ba17482f 100644
--- a/src/reader2/ISyntaxStateMachine_DToplevelSeqSsm.cpp
+++ b/src/reader2/ISyntaxStateMachine_DToplevelSeqSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DToplevelSeqSsm::visit_gco_children(DToplevelSeqSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DToplevelSeqSsm::visit_gco_children(DToplevelSeqSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/ParserResult.cpp b/src/reader2/ParserResult.cpp
index 609f204f..bb3d4034 100644
--- a/src/reader2/ParserResult.cpp
+++ b/src/reader2/ParserResult.cpp
@@ -106,12 +106,13 @@ namespace xo {
         }
 
         void
-        ParserResult::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        ParserResult::visit_gco_children(VisitReason reason,
+                                         obj<AGCObjectVisitor> gc) noexcept
         {
             // {result_type_, error_src_fn_}: pod, ignore
 
-            gc.visit_poly_child(&result_expr_);
-            gc.visit_child(&error_description_);
+            gc.visit_poly_child(reason, &result_expr_);
+            gc.visit_child(reason, &error_description_);
         }
     } /*namespace scm*/
 } /*namespace xo*/
diff --git a/src/reader2/ParserStack.cpp b/src/reader2/ParserStack.cpp
index 721b19b4..83d21df1 100644
--- a/src/reader2/ParserStack.cpp
+++ b/src/reader2/ParserStack.cpp
@@ -90,14 +90,15 @@ namespace xo {
         }
 
         void
-        ParserStack::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        ParserStack::visit_gco_children(VisitReason reason,
+                                        obj<AGCObjectVisitor> gc) noexcept
         {
 
             for (ParserStack * target = this; target; target = target->parent_) {
                 // ParserStack::ckp: skip, POD
 
                 if (target->ssm_)
-                    target->ssm_.visit_gco_children(gc);
+                    target->ssm_.visit_gco_children(reason, gc);
             }
         }
 
diff --git a/src/reader2/ParserStateMachine.cpp b/src/reader2/ParserStateMachine.cpp
index 6bec1833..71028ebe 100644
--- a/src/reader2/ParserStateMachine.cpp
+++ b/src/reader2/ParserStateMachine.cpp
@@ -907,7 +907,8 @@ namespace xo {
 #endif
 
         void
-        ParserStateMachine::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        ParserStateMachine::visit_gco_children(VisitReason reason,
+                                               obj<AGCObjectVisitor> gc) noexcept
         {
             //scope log(XO_DEBUG(true));
 
@@ -916,23 +917,23 @@ namespace xo {
 
             //log && log("forward stack_", xtag("addr", stack_));
             if (stack_) {
-                stack_->visit_gco_children(gc);
+                stack_->visit_gco_children(reason, gc);
             }
 
             // static_assert(!expr_alloc_.is_gc_eligible());
             // static_assert(!aux_alloc_.is_gc_eligible());
 
             //log && log("global_symtab_", xtag("addr", global_symtab_.data()));
-            gc.visit_child(&global_symtab_);
+            gc.visit_child(reason, &global_symtab_);
 
             //log && log("local_symtab_", xtag("addr", local_symtab_.data()));
-            gc.visit_child(&local_symtab_);
+            gc.visit_child(reason, &local_symtab_);
 
             //log && log("global_env_", xtag("addr", global_env_.data()));
-            gc.visit_child(&global_env_);
+            gc.visit_child(reason, &global_env_);
 
             //log && log("result_");
-            result_.visit_gco_children(gc);
+            result_.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/SchematikaReader.cpp b/src/reader2/SchematikaReader.cpp
index c9405916..168f562a 100644
--- a/src/reader2/SchematikaReader.cpp
+++ b/src/reader2/SchematikaReader.cpp
@@ -10,9 +10,10 @@ namespace xo {
 
     namespace scm {
         void
-        ReaderResult::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        ReaderResult::visit_gco_children(VisitReason reason,
+                                         obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.visit_poly_child(&expr_);
+            gc.visit_poly_child(reason, &expr_);
         }
 
         // ----- SchematikaReader -----
@@ -208,12 +209,13 @@ namespace xo {
         }
 
         void
-        SchematikaReader::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
+        SchematikaReader::visit_gco_children(VisitReason reason,
+                                             obj<AGCObjectVisitor> gc) noexcept
         {
             // tokenizer doesn't contain any gc-aware pointers.
 
-            parser_.visit_gco_children(gc);
-            result_.visit_gco_children(gc);
+            parser_.visit_gco_children(reason, gc);
+            result_.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/facet/IGCObject_DGlobalEnv.cpp b/src/reader2/facet/IGCObject_DGlobalEnv.cpp
index 9a2802ae..72c9b065 100644
--- a/src/reader2/facet/IGCObject_DGlobalEnv.cpp
+++ b/src/reader2/facet/IGCObject_DGlobalEnv.cpp
@@ -21,9 +21,9 @@ namespace xo {
             return self.gco_shallow_move(gc);
         }
         auto
-        IGCObject_DGlobalEnv::visit_gco_children(DGlobalEnv & self, obj<AGCObjectVisitor> fn) noexcept -> void
+        IGCObject_DGlobalEnv::visit_gco_children(DGlobalEnv & self, VisitReason reason, obj<AGCObjectVisitor> fn) noexcept -> void
         {
-            self.visit_gco_children(fn);
+            self.visit_gco_children(reason, fn);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/facet/IGCObject_DSchematikaParser.cpp b/src/reader2/facet/IGCObject_DSchematikaParser.cpp
index f1778d5a..7586fc25 100644
--- a/src/reader2/facet/IGCObject_DSchematikaParser.cpp
+++ b/src/reader2/facet/IGCObject_DSchematikaParser.cpp
@@ -21,9 +21,9 @@ namespace xo {
             return self.gco_shallow_move(gc);
         }
         auto
-        IGCObject_DSchematikaParser::visit_gco_children(DSchematikaParser & self, obj<AGCObjectVisitor> fn) noexcept -> void
+        IGCObject_DSchematikaParser::visit_gco_children(DSchematikaParser & self, VisitReason reason, obj<AGCObjectVisitor> fn) noexcept -> void
         {
-            self.visit_gco_children(fn);
+            self.visit_gco_children(reason, fn);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/facet/ISyntaxStateMachine_DApplySsm.cpp b/src/reader2/facet/ISyntaxStateMachine_DApplySsm.cpp
index 01f685fe..baba1c7f 100644
--- a/src/reader2/facet/ISyntaxStateMachine_DApplySsm.cpp
+++ b/src/reader2/facet/ISyntaxStateMachine_DApplySsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DApplySsm::visit_gco_children(DApplySsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DApplySsm::visit_gco_children(DApplySsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/facet/ISyntaxStateMachine_DDefineSsm.cpp b/src/reader2/facet/ISyntaxStateMachine_DDefineSsm.cpp
index 886155c0..5fe37536 100644
--- a/src/reader2/facet/ISyntaxStateMachine_DDefineSsm.cpp
+++ b/src/reader2/facet/ISyntaxStateMachine_DDefineSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DDefineSsm::visit_gco_children(DDefineSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DDefineSsm::visit_gco_children(DDefineSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/facet/ISyntaxStateMachine_DDeftypeSsm.cpp b/src/reader2/facet/ISyntaxStateMachine_DDeftypeSsm.cpp
index 874849ff..a88b1005 100644
--- a/src/reader2/facet/ISyntaxStateMachine_DDeftypeSsm.cpp
+++ b/src/reader2/facet/ISyntaxStateMachine_DDeftypeSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DDeftypeSsm::visit_gco_children(DDeftypeSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DDeftypeSsm::visit_gco_children(DDeftypeSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/facet/ISyntaxStateMachine_DExpectFormalArgSsm.cpp b/src/reader2/facet/ISyntaxStateMachine_DExpectFormalArgSsm.cpp
index 85f8dd7a..c0adc804 100644
--- a/src/reader2/facet/ISyntaxStateMachine_DExpectFormalArgSsm.cpp
+++ b/src/reader2/facet/ISyntaxStateMachine_DExpectFormalArgSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DExpectFormalArgSsm::visit_gco_children(DExpectFormalArgSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DExpectFormalArgSsm::visit_gco_children(DExpectFormalArgSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/facet/ISyntaxStateMachine_DExpectListTypeSsm.cpp b/src/reader2/facet/ISyntaxStateMachine_DExpectListTypeSsm.cpp
index d3b292ad..1306e061 100644
--- a/src/reader2/facet/ISyntaxStateMachine_DExpectListTypeSsm.cpp
+++ b/src/reader2/facet/ISyntaxStateMachine_DExpectListTypeSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DExpectListTypeSsm::visit_gco_children(DExpectListTypeSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DExpectListTypeSsm::visit_gco_children(DExpectListTypeSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/facet/ISyntaxStateMachine_DExpectQDictSsm.cpp b/src/reader2/facet/ISyntaxStateMachine_DExpectQDictSsm.cpp
index 9f4d74cc..5cc75b00 100644
--- a/src/reader2/facet/ISyntaxStateMachine_DExpectQDictSsm.cpp
+++ b/src/reader2/facet/ISyntaxStateMachine_DExpectQDictSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DExpectQDictSsm::visit_gco_children(DExpectQDictSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DExpectQDictSsm::visit_gco_children(DExpectQDictSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/facet/ISyntaxStateMachine_DIfElseSsm.cpp b/src/reader2/facet/ISyntaxStateMachine_DIfElseSsm.cpp
index 68034ba4..37a20363 100644
--- a/src/reader2/facet/ISyntaxStateMachine_DIfElseSsm.cpp
+++ b/src/reader2/facet/ISyntaxStateMachine_DIfElseSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DIfElseSsm::visit_gco_children(DIfElseSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DIfElseSsm::visit_gco_children(DIfElseSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/facet/ISyntaxStateMachine_DLambdaSsm.cpp b/src/reader2/facet/ISyntaxStateMachine_DLambdaSsm.cpp
index 88e52100..08754857 100644
--- a/src/reader2/facet/ISyntaxStateMachine_DLambdaSsm.cpp
+++ b/src/reader2/facet/ISyntaxStateMachine_DLambdaSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DLambdaSsm::visit_gco_children(DLambdaSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DLambdaSsm::visit_gco_children(DLambdaSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/
diff --git a/src/reader2/facet/ISyntaxStateMachine_DQuoteSsm.cpp b/src/reader2/facet/ISyntaxStateMachine_DQuoteSsm.cpp
index 39d26414..b4ee1261 100644
--- a/src/reader2/facet/ISyntaxStateMachine_DQuoteSsm.cpp
+++ b/src/reader2/facet/ISyntaxStateMachine_DQuoteSsm.cpp
@@ -78,9 +78,9 @@ namespace xo {
             self.on_quoted_literal(lit, p_psm);
         }
         auto
-        ISyntaxStateMachine_DQuoteSsm::visit_gco_children(DQuoteSsm & self, obj<AGCObjectVisitor> gc) -> void
+        ISyntaxStateMachine_DQuoteSsm::visit_gco_children(DQuoteSsm & self, VisitReason reason, obj<AGCObjectVisitor> gc) -> void
         {
-            self.visit_gco_children(gc);
+            self.visit_gco_children(reason, gc);
         }
 
     } /*namespace scm*/