diff --git a/include/xo/interpreter2/DClosure.hpp b/include/xo/interpreter2/DClosure.hpp
index e57f6a1a..4d24d72e 100644
--- a/include/xo/interpreter2/DClosure.hpp
+++ b/include/xo/interpreter2/DClosure.hpp
@@ -20,8 +20,9 @@ namespace xo {
         public:
             using ARuntimeContext = xo::scm::ARuntimeContext;
             using ACollector = xo::mm::ACollector;
-            using AAllocator = xo::mm::AAllocator;
             using AGCObject = xo::mm::AGCObject;
+            using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using AAllocator = xo::mm::AAllocator;
             using ppindentinfo = xo::print::ppindentinfo;
             using size_type = std::int32_t;
 
@@ -57,9 +58,8 @@ namespace xo {
             /** @defgroup scm-closure-gcobject-facet **/
             ///@{
 
-            std::size_t shallow_size() const noexcept;
             DClosure * shallow_move(obj<ACollector> gc) noexcept;
-            std::size_t forward_children(obj<ACollector> gc) noexcept;
+            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
             /** @defgroup scm-closure-printable-facet **/
diff --git a/include/xo/interpreter2/DLocalEnv.hpp b/include/xo/interpreter2/DLocalEnv.hpp
index 6ecd3eb6..5270dec1 100644
--- a/include/xo/interpreter2/DLocalEnv.hpp
+++ b/include/xo/interpreter2/DLocalEnv.hpp
@@ -17,8 +17,9 @@ namespace xo {
         public:
             using DArray = xo::scm::DArray;
             using ACollector = xo::mm::ACollector;
-            using AAllocator = xo::mm::AAllocator;
             using AGCObject = xo::mm::AGCObject;
+            using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using AAllocator = xo::mm::AAllocator;
             using ppindentinfo = xo::print::ppindentinfo;
             using size_type = std::uint32_t;
 
@@ -54,9 +55,8 @@ namespace xo {
             /** @defgroup scm-localenv-gcobject-facet **/
             ///@{
 
-            std::size_t shallow_size() const noexcept;
             DLocalEnv * shallow_move(obj<ACollector> gc) noexcept;
-            std::size_t forward_children(obj<ACollector> gc) noexcept;
+            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
             /** @defgroup scm-localenv-printable-facet **/
diff --git a/include/xo/interpreter2/DVsmApplyClosureFrame.hpp b/include/xo/interpreter2/DVsmApplyClosureFrame.hpp
index ae422cb1..8dd30efb 100644
--- a/include/xo/interpreter2/DVsmApplyClosureFrame.hpp
+++ b/include/xo/interpreter2/DVsmApplyClosureFrame.hpp
@@ -19,8 +19,9 @@ namespace xo {
         class DVsmApplyClosureFrame {
         public:
             using ACollector = xo::mm::ACollector;
-            using AAllocator = xo::mm::AAllocator;
             using AGCObject = xo::mm::AGCObject;
+            using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using AAllocator = xo::mm::AAllocator;
             using ppindentinfo = xo::print::ppindentinfo;
 
         public:
@@ -41,7 +42,7 @@ namespace xo {
             /** gcobject facet **/
             std::size_t shallow_size() const noexcept;
             DVsmApplyClosureFrame * shallow_move(obj<ACollector> gc) noexcept;
-            std::size_t forward_children(obj<ACollector> gc) noexcept;
+            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
 
             /** pretty-printing support **/
             bool pretty(const ppindentinfo & ppii) const;
diff --git a/include/xo/interpreter2/DVsmApplyFrame.hpp b/include/xo/interpreter2/DVsmApplyFrame.hpp
index 99093f37..d8e9ab79 100644
--- a/include/xo/interpreter2/DVsmApplyFrame.hpp
+++ b/include/xo/interpreter2/DVsmApplyFrame.hpp
@@ -15,8 +15,9 @@ namespace xo {
         public:
             using AProcedure = xo::scm::AProcedure;
             using ACollector = xo::mm::ACollector;
-            using AAllocator = xo::mm::AAllocator;
             using AGCObject = xo::mm::AGCObject;
+            using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using AAllocator = xo::mm::AAllocator;
             using ppindentinfo = xo::print::ppindentinfo;
 
         public:
@@ -37,9 +38,8 @@ namespace xo {
 
             void assign_fn(obj<AGCObject> x) { this->fn_ = x; }
 
-            std::size_t shallow_size() const noexcept;
             DVsmApplyFrame * shallow_move(obj<ACollector> gc) noexcept;
-            std::size_t forward_children(obj<ACollector> gc) noexcept;
+            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
 
             /** pretty-printing support **/
             bool pretty(const ppindentinfo & ppii) const;
diff --git a/include/xo/interpreter2/DVsmDefContFrame.hpp b/include/xo/interpreter2/DVsmDefContFrame.hpp
index 0ad31cca..acdd8a90 100644
--- a/include/xo/interpreter2/DVsmDefContFrame.hpp
+++ b/include/xo/interpreter2/DVsmDefContFrame.hpp
@@ -16,6 +16,7 @@ namespace xo {
         class DVsmDefContFrame {
         public:
             using ACollector = xo::mm::ACollector;
+            using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
             using AAllocator = xo::mm::AAllocator;
             using AGCObject = xo::mm::AGCObject;
             using ppindentinfo = xo::print::ppindentinfo;
@@ -50,9 +51,8 @@ namespace xo {
             /** @defgroup scm-vsmdefcontframe-gcobject-facet gcobject facet **/
             ///@{
 
-            std::size_t shallow_size() const noexcept;
             DVsmDefContFrame * shallow_move(obj<ACollector> gc) noexcept;
-            std::size_t forward_children(obj<ACollector> gc) noexcept;
+            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
             /** @defgrouop scm-vsmseqcontframe-printable-facet printable facet **/
diff --git a/include/xo/interpreter2/DVsmEvalArgsFrame.hpp b/include/xo/interpreter2/DVsmEvalArgsFrame.hpp
index a1903e94..ab7a4607 100644
--- a/include/xo/interpreter2/DVsmEvalArgsFrame.hpp
+++ b/include/xo/interpreter2/DVsmEvalArgsFrame.hpp
@@ -15,8 +15,9 @@ namespace xo {
         class DVsmEvalArgsFrame {
         public:
             using ACollector = xo::mm::ACollector;
-            using AAllocator = xo::mm::AAllocator;
             using AGCObject = xo::mm::AGCObject;
+            using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using AAllocator = xo::mm::AAllocator;
             using ppindentinfo = xo::print::ppindentinfo;
 
         public:
@@ -42,9 +43,8 @@ namespace xo {
 
             int32_t increment_arg() { return ++i_arg_; }
 
-            std::size_t shallow_size() const noexcept;
             DVsmEvalArgsFrame * shallow_move(obj<ACollector> gc) noexcept;
-            std::size_t forward_children(obj<ACollector> gc) noexcept;
+            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
 
             bool pretty(const ppindentinfo & ppii) const;
 
diff --git a/include/xo/interpreter2/DVsmIfElseContFrame.hpp b/include/xo/interpreter2/DVsmIfElseContFrame.hpp
index 3c438e51..82c5ee31 100644
--- a/include/xo/interpreter2/DVsmIfElseContFrame.hpp
+++ b/include/xo/interpreter2/DVsmIfElseContFrame.hpp
@@ -16,8 +16,9 @@ namespace xo {
         class DVsmIfElseContFrame {
         public:
             using ACollector = xo::mm::ACollector;
-            using AAllocator = xo::mm::AAllocator;
+            using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
             using AGCObject = xo::mm::AGCObject;
+            using AAllocator = xo::mm::AAllocator;
             using ppindentinfo = xo::print::ppindentinfo;
 
         public:
@@ -50,9 +51,8 @@ namespace xo {
             /** @defgroup scm-vsmevalsequenceframe-gcobject-facet gcobject facet **/
             ///@{
 
-            std::size_t shallow_size() const noexcept;
             DVsmIfElseContFrame * shallow_move(obj<ACollector> gc) noexcept;
-            std::size_t forward_children(obj<ACollector> gc) noexcept;
+            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
             /** @defgrouop scm-vsmseqcontframe-printable-facet printable facet **/
diff --git a/include/xo/interpreter2/DVsmSeqContFrame.hpp b/include/xo/interpreter2/DVsmSeqContFrame.hpp
index 959edfb0..65532996 100644
--- a/include/xo/interpreter2/DVsmSeqContFrame.hpp
+++ b/include/xo/interpreter2/DVsmSeqContFrame.hpp
@@ -18,6 +18,7 @@ namespace xo {
             using ACollector = xo::mm::ACollector;
             using AAllocator = xo::mm::AAllocator;
             using AGCObject = xo::mm::AGCObject;
+            using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
             using ppindentinfo = xo::print::ppindentinfo;
 
         public:
@@ -55,9 +56,8 @@ namespace xo {
             /** @defgroup scm-vsmevalsequenceframe-gcobject-facet gcobject facet **/
             ///@{
 
-            std::size_t shallow_size() const noexcept;
             DVsmSeqContFrame * shallow_move(obj<ACollector> gc) noexcept;
-            std::size_t forward_children(obj<ACollector> gc) noexcept;
+            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
             /** @defgrouop scm-vsmseqcontframe-printable-facet printable facet **/
diff --git a/include/xo/interpreter2/define/IGCObject_DVsmDefContFrame.hpp b/include/xo/interpreter2/define/IGCObject_DVsmDefContFrame.hpp
index 7d082c38..85949311 100644
--- a/include/xo/interpreter2/define/IGCObject_DVsmDefContFrame.hpp
+++ b/include/xo/interpreter2/define/IGCObject_DVsmDefContFrame.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using size_type = xo::mm::AGCObject::size_type;
             using AAllocator = xo::mm::AGCObject::AAllocator;
             using ACollector = xo::mm::AGCObject::ACollector;
+            using AGCObjectVisitor = xo::mm::AGCObject::AGCObjectVisitor;
             using Copaque = xo::mm::AGCObject::Copaque;
             using Opaque = xo::mm::AGCObject::Opaque;
             ///@}
@@ -54,8 +55,10 @@ namespace xo {
             // non-const methods
             /** move instance using allocator **/
             static Opaque shallow_move(DVsmDefContFrame & self, obj<ACollector> gc) noexcept;
-            /** during GC: forward immdiate children **/
-            static void forward_children(DVsmDefContFrame & self, obj<ACollector> gc) noexcept;
+            /** 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(DVsmDefContFrame & self, obj<AGCObjectVisitor> fn) noexcept;
             ///@}
         };
 
diff --git a/include/xo/interpreter2/detail/IGCObject_DClosure.hpp b/include/xo/interpreter2/detail/IGCObject_DClosure.hpp
index 84502baa..284199eb 100644
--- a/include/xo/interpreter2/detail/IGCObject_DClosure.hpp
+++ b/include/xo/interpreter2/detail/IGCObject_DClosure.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using size_type = xo::mm::AGCObject::size_type;
             using AAllocator = xo::mm::AGCObject::AAllocator;
             using ACollector = xo::mm::AGCObject::ACollector;
+            using AGCObjectVisitor = xo::mm::AGCObject::AGCObjectVisitor;
             using Copaque = xo::mm::AGCObject::Copaque;
             using Opaque = xo::mm::AGCObject::Opaque;
             ///@}
@@ -54,8 +55,10 @@ namespace xo {
             // non-const methods
             /** move instance using allocator **/
             static Opaque shallow_move(DClosure & self, obj<ACollector> gc) noexcept;
-            /** during GC: forward immdiate children **/
-            static void forward_children(DClosure & self, obj<ACollector> gc) noexcept;
+            /** 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(DClosure & self, obj<AGCObjectVisitor> fn) noexcept;
             ///@}
         };
 
diff --git a/include/xo/interpreter2/detail/IGCObject_DVsmApplyClosureFrame.hpp b/include/xo/interpreter2/detail/IGCObject_DVsmApplyClosureFrame.hpp
index 9a2e478d..b6a92890 100644
--- a/include/xo/interpreter2/detail/IGCObject_DVsmApplyClosureFrame.hpp
+++ b/include/xo/interpreter2/detail/IGCObject_DVsmApplyClosureFrame.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using size_type = xo::mm::AGCObject::size_type;
             using AAllocator = xo::mm::AGCObject::AAllocator;
             using ACollector = xo::mm::AGCObject::ACollector;
+            using AGCObjectVisitor = xo::mm::AGCObject::AGCObjectVisitor;
             using Copaque = xo::mm::AGCObject::Copaque;
             using Opaque = xo::mm::AGCObject::Opaque;
             ///@}
@@ -54,8 +55,10 @@ namespace xo {
             // non-const methods
             /** move instance using allocator **/
             static Opaque shallow_move(DVsmApplyClosureFrame & self, obj<ACollector> gc) noexcept;
-            /** during GC: forward immdiate children **/
-            static void forward_children(DVsmApplyClosureFrame & self, obj<ACollector> gc) noexcept;
+            /** 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(DVsmApplyClosureFrame & self, obj<AGCObjectVisitor> fn) noexcept;
             ///@}
         };
 
diff --git a/include/xo/interpreter2/detail/IGCObject_DVsmApplyFrame.hpp b/include/xo/interpreter2/detail/IGCObject_DVsmApplyFrame.hpp
index 9de1e8f3..85b3f614 100644
--- a/include/xo/interpreter2/detail/IGCObject_DVsmApplyFrame.hpp
+++ b/include/xo/interpreter2/detail/IGCObject_DVsmApplyFrame.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using size_type = xo::mm::AGCObject::size_type;
             using AAllocator = xo::mm::AGCObject::AAllocator;
             using ACollector = xo::mm::AGCObject::ACollector;
+            using AGCObjectVisitor = xo::mm::AGCObject::AGCObjectVisitor;
             using Copaque = xo::mm::AGCObject::Copaque;
             using Opaque = xo::mm::AGCObject::Opaque;
             ///@}
@@ -54,8 +55,10 @@ namespace xo {
             // non-const methods
             /** move instance using allocator **/
             static Opaque shallow_move(DVsmApplyFrame & self, obj<ACollector> gc) noexcept;
-            /** during GC: forward immdiate children **/
-            static void forward_children(DVsmApplyFrame & self, obj<ACollector> gc) noexcept;
+            /** 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(DVsmApplyFrame & self, obj<AGCObjectVisitor> fn) noexcept;
             ///@}
         };
 
diff --git a/include/xo/interpreter2/detail/IGCObject_DVsmEvalArgsFrame.hpp b/include/xo/interpreter2/detail/IGCObject_DVsmEvalArgsFrame.hpp
index 9e2841a6..d62489c3 100644
--- a/include/xo/interpreter2/detail/IGCObject_DVsmEvalArgsFrame.hpp
+++ b/include/xo/interpreter2/detail/IGCObject_DVsmEvalArgsFrame.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using size_type = xo::mm::AGCObject::size_type;
             using AAllocator = xo::mm::AGCObject::AAllocator;
             using ACollector = xo::mm::AGCObject::ACollector;
+            using AGCObjectVisitor = xo::mm::AGCObject::AGCObjectVisitor;
             using Copaque = xo::mm::AGCObject::Copaque;
             using Opaque = xo::mm::AGCObject::Opaque;
             ///@}
@@ -54,8 +55,10 @@ namespace xo {
             // non-const methods
             /** move instance using allocator **/
             static Opaque shallow_move(DVsmEvalArgsFrame & self, obj<ACollector> gc) noexcept;
-            /** during GC: forward immdiate children **/
-            static void forward_children(DVsmEvalArgsFrame & self, obj<ACollector> gc) noexcept;
+            /** 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(DVsmEvalArgsFrame & self, obj<AGCObjectVisitor> fn) noexcept;
             ///@}
         };
 
diff --git a/include/xo/interpreter2/env/IGCObject_DLocalEnv.hpp b/include/xo/interpreter2/env/IGCObject_DLocalEnv.hpp
index e4de912e..599829a9 100644
--- a/include/xo/interpreter2/env/IGCObject_DLocalEnv.hpp
+++ b/include/xo/interpreter2/env/IGCObject_DLocalEnv.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using size_type = xo::mm::AGCObject::size_type;
             using AAllocator = xo::mm::AGCObject::AAllocator;
             using ACollector = xo::mm::AGCObject::ACollector;
+            using AGCObjectVisitor = xo::mm::AGCObject::AGCObjectVisitor;
             using Copaque = xo::mm::AGCObject::Copaque;
             using Opaque = xo::mm::AGCObject::Opaque;
             ///@}
@@ -54,8 +55,10 @@ namespace xo {
             // non-const methods
             /** move instance using allocator **/
             static Opaque shallow_move(DLocalEnv & self, obj<ACollector> gc) noexcept;
-            /** during GC: forward immdiate children **/
-            static void forward_children(DLocalEnv & self, obj<ACollector> gc) noexcept;
+            /** 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(DLocalEnv & self, obj<AGCObjectVisitor> fn) noexcept;
             ///@}
         };
 
diff --git a/include/xo/interpreter2/ifelse/IGCObject_DVsmIfElseContFrame.hpp b/include/xo/interpreter2/ifelse/IGCObject_DVsmIfElseContFrame.hpp
index a3cede47..c0c25c65 100644
--- a/include/xo/interpreter2/ifelse/IGCObject_DVsmIfElseContFrame.hpp
+++ b/include/xo/interpreter2/ifelse/IGCObject_DVsmIfElseContFrame.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using size_type = xo::mm::AGCObject::size_type;
             using AAllocator = xo::mm::AGCObject::AAllocator;
             using ACollector = xo::mm::AGCObject::ACollector;
+            using AGCObjectVisitor = xo::mm::AGCObject::AGCObjectVisitor;
             using Copaque = xo::mm::AGCObject::Copaque;
             using Opaque = xo::mm::AGCObject::Opaque;
             ///@}
@@ -54,8 +55,10 @@ namespace xo {
             // non-const methods
             /** move instance using allocator **/
             static Opaque shallow_move(DVsmIfElseContFrame & self, obj<ACollector> gc) noexcept;
-            /** during GC: forward immdiate children **/
-            static void forward_children(DVsmIfElseContFrame & self, obj<ACollector> gc) noexcept;
+            /** 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(DVsmIfElseContFrame & self, obj<AGCObjectVisitor> fn) noexcept;
             ///@}
         };
 
diff --git a/include/xo/interpreter2/sequence/IGCObject_DVsmSeqContFrame.hpp b/include/xo/interpreter2/sequence/IGCObject_DVsmSeqContFrame.hpp
index 14367b50..0c9fe148 100644
--- a/include/xo/interpreter2/sequence/IGCObject_DVsmSeqContFrame.hpp
+++ b/include/xo/interpreter2/sequence/IGCObject_DVsmSeqContFrame.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using size_type = xo::mm::AGCObject::size_type;
             using AAllocator = xo::mm::AGCObject::AAllocator;
             using ACollector = xo::mm::AGCObject::ACollector;
+            using AGCObjectVisitor = xo::mm::AGCObject::AGCObjectVisitor;
             using Copaque = xo::mm::AGCObject::Copaque;
             using Opaque = xo::mm::AGCObject::Opaque;
             ///@}
@@ -54,8 +55,10 @@ namespace xo {
             // non-const methods
             /** move instance using allocator **/
             static Opaque shallow_move(DVsmSeqContFrame & self, obj<ACollector> gc) noexcept;
-            /** during GC: forward immdiate children **/
-            static void forward_children(DVsmSeqContFrame & self, obj<ACollector> gc) noexcept;
+            /** 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(DVsmSeqContFrame & self, obj<AGCObjectVisitor> fn) noexcept;
             ///@}
         };
 
diff --git a/include/xo/interpreter2/vsm/DVirtualSchematikaMachine.hpp b/include/xo/interpreter2/vsm/DVirtualSchematikaMachine.hpp
index fa9a6929..28343528 100644
--- a/include/xo/interpreter2/vsm/DVirtualSchematikaMachine.hpp
+++ b/include/xo/interpreter2/vsm/DVirtualSchematikaMachine.hpp
@@ -72,8 +72,9 @@ namespace xo {
             // will be DArenaVector<obj<StackFrame>> probably
             using Stack = void *;
             using ACollector = xo::mm::ACollector;
-            using AAllocator = xo::mm::AAllocator;
             using AGCObject = xo::mm::AGCObject;
+            using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+            using AAllocator = xo::mm::AAllocator;
             using MemorySizeVisitor = xo::mm::MemorySizeVisitor;
             using span_type = xo::mm::span<const char>;
 
@@ -157,7 +158,7 @@ namespace xo {
 
             /** forward gc-aware child pointers
              **/
-            std::size_t forward_children(obj<ACollector> gc) noexcept;
+            void visit_gco_children(obj<AGCObjectVisitor> gc) noexcept;
 
             ///@}
 
diff --git a/include/xo/interpreter2/vsm/IGCObject_DVirtualSchematikaMachine.hpp b/include/xo/interpreter2/vsm/IGCObject_DVirtualSchematikaMachine.hpp
index 073e9fac..3d72b7af 100644
--- a/include/xo/interpreter2/vsm/IGCObject_DVirtualSchematikaMachine.hpp
+++ b/include/xo/interpreter2/vsm/IGCObject_DVirtualSchematikaMachine.hpp
@@ -44,6 +44,7 @@ namespace xo {
             using size_type = xo::mm::AGCObject::size_type;
             using AAllocator = xo::mm::AGCObject::AAllocator;
             using ACollector = xo::mm::AGCObject::ACollector;
+            using AGCObjectVisitor = xo::mm::AGCObject::AGCObjectVisitor;
             using Copaque = xo::mm::AGCObject::Copaque;
             using Opaque = xo::mm::AGCObject::Opaque;
             ///@}
@@ -54,8 +55,10 @@ namespace xo {
             // non-const methods
             /** move instance using allocator **/
             static Opaque shallow_move(DVirtualSchematikaMachine & self, obj<ACollector> gc) noexcept;
-            /** during GC: forward immdiate children **/
-            static void forward_children(DVirtualSchematikaMachine & self, obj<ACollector> gc) noexcept;
+            /** 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(DVirtualSchematikaMachine & self, obj<AGCObjectVisitor> fn) noexcept;
             ///@}
         };
 
diff --git a/src/interpreter2/DClosure.cpp b/src/interpreter2/DClosure.cpp
index 6b346463..ea8bcb15 100644
--- a/src/interpreter2/DClosure.cpp
+++ b/src/interpreter2/DClosure.cpp
@@ -64,32 +64,16 @@ namespace xo {
             return err;
         }
 
-        size_t
-        DClosure::shallow_size() const noexcept {
-            return sizeof(DClosure);
-        }
-
         DClosure *
         DClosure::shallow_move(obj<ACollector> gc) noexcept {
             return gc.std_move_for(this);
         }
 
-        std::size_t
-        DClosure::forward_children(obj<ACollector> gc) noexcept
+        void
+        DClosure::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
         {
-            {
-                gc.forward_inplace(&lambda_);
-                //auto iface = xo::facet::impl_for<AGCObject,DLambdaExpr>();
-                //gc.forward_inplace(&iface, (void **)(&lambda_));
-
-            }
-            {
-                gc.forward_inplace(&env_);
-                //auto iface = xo::facet::impl_for<AGCObject,DLocalEnv>();
-                //gc.forward_inplace(&iface, (void **)(&env_));
-            }
-
-            return this->shallow_size();
+            gc.visit_child(&lambda_);
+            gc.visit_child(&env_);
         }
 
         // ----- printable facet -----
diff --git a/src/interpreter2/DLocalEnv.cpp b/src/interpreter2/DLocalEnv.cpp
index 4124e280..3da6fd9b 100644
--- a/src/interpreter2/DLocalEnv.cpp
+++ b/src/interpreter2/DLocalEnv.cpp
@@ -91,33 +91,29 @@ namespace xo {
             /* something terribly wrong if control here */
         }
 
-        std::size_t
-        DLocalEnv::shallow_size() const noexcept {
-            return sizeof(DLocalEnv);
-        }
-
         DLocalEnv *
         DLocalEnv::shallow_move(obj<ACollector> gc) noexcept {
             return gc.std_move_for(this);
         }
 
-        std::size_t
-        DLocalEnv::forward_children(obj<ACollector> gc) noexcept
+        void
+        DLocalEnv::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
         {
             {
-                auto iface = xo::facet::impl_for<AGCObject,DLocalEnv>();
-                gc.forward_inplace(&iface, (void **)(&parent_));
+                gc.visit_child(&parent_);
+                //auto iface = xo::facet::impl_for<AGCObject,DLocalEnv>();
+                //gc.forward_inplace(&iface, (void **)(&parent_));
             }
             {
-                auto iface = xo::facet::impl_for<AGCObject,DLocalSymtab>();
-                gc.forward_inplace(&iface, (void **)(&symtab_));
+                gc.visit_child(&symtab_);
+                //auto iface = xo::facet::impl_for<AGCObject,DLocalSymtab>();
+                //gc.forward_inplace(&iface, (void **)(&symtab_));
             }
             {
-                auto iface = xo::facet::impl_for<AGCObject,DArray>();
-                gc.forward_inplace(&iface, (void **)(&args_));
+                gc.visit_child(&args_);
+                //auto iface = xo::facet::impl_for<AGCObject,DArray>();
+                //gc.forward_inplace(&iface, (void **)(&args_));
             }
-
-            return shallow_size();
         }
 
         // ----- printable facet -----
diff --git a/src/interpreter2/DVirtualSchematikaMachine.cpp b/src/interpreter2/DVirtualSchematikaMachine.cpp
index 06a4c23f..8f9d3967 100644
--- a/src/interpreter2/DVirtualSchematikaMachine.cpp
+++ b/src/interpreter2/DVirtualSchematikaMachine.cpp
@@ -968,22 +968,20 @@ namespace xo {
             return nullptr;
         }
 
-        std::size_t
-        DVirtualSchematikaMachine::forward_children(obj<ACollector> gc) noexcept
+        void
+        DVirtualSchematikaMachine::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
         {
-            reader_.forward_children(gc);
+            reader_.visit_gco_children(gc);
 
-            gc.forward_inplace(&stack_);
-            gc.forward_pivot_inplace(&expr_);
-            gc.forward_inplace(&global_env_);
-            gc.forward_inplace(&local_env_);
-            gc.forward_inplace(&fn_);
-            gc.forward_inplace(&args_);
+            gc.visit_child(&stack_);
+            gc.visit_poly_child(&expr_);
+            gc.visit_child(&global_env_);
+            gc.visit_child(&local_env_);
+            gc.visit_child(&fn_);
+            gc.visit_child(&args_);
             if (value_.is_value()) {
-                gc.forward_inplace(const_cast<obj<AGCObject> *>(&value_.value_ref()));
+                gc.visit_child(const_cast<obj<AGCObject> *>(&value_.value_ref()));
             }
-
-            return this->shallow_size();
         }
 
     } /*namespace scm*/
diff --git a/src/interpreter2/DVsmApplyClosureFrame.cpp b/src/interpreter2/DVsmApplyClosureFrame.cpp
index 129d0d46..c17414b8 100644
--- a/src/interpreter2/DVsmApplyClosureFrame.cpp
+++ b/src/interpreter2/DVsmApplyClosureFrame.cpp
@@ -32,25 +32,17 @@ namespace xo {
             return new (mem) DVsmApplyClosureFrame(stack, cont, local_env);
         }
 
-        std::size_t
-        DVsmApplyClosureFrame::shallow_size() const noexcept
-        {
-            return sizeof(DVsmApplyClosureFrame);
-        }
-
         DVsmApplyClosureFrame *
         DVsmApplyClosureFrame::shallow_move(obj<ACollector> gc) noexcept
         {
             return gc.std_move_for(this);
         }
 
-        std::size_t
-        DVsmApplyClosureFrame::forward_children(obj<ACollector> gc) noexcept
+        void
+        DVsmApplyClosureFrame::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.forward_inplace(&stack_);
-            gc.forward_inplace(&local_env_);
-
-            return this->shallow_size();
+            gc.visit_child(&stack_);
+            gc.visit_child(&local_env_);
         }
 
         bool
diff --git a/src/interpreter2/DVsmApplyFrame.cpp b/src/interpreter2/DVsmApplyFrame.cpp
index d388f9c4..1a9a946d 100644
--- a/src/interpreter2/DVsmApplyFrame.cpp
+++ b/src/interpreter2/DVsmApplyFrame.cpp
@@ -40,26 +40,18 @@ namespace xo {
             return result;
         }
 
-        std::size_t
-        DVsmApplyFrame::shallow_size() const noexcept
-        {
-            return sizeof(DVsmApplyFrame);
-        }
-
         DVsmApplyFrame *
         DVsmApplyFrame::shallow_move(obj<ACollector> gc) noexcept
         {
             return gc.std_move_for(this);
         }
 
-        std::size_t
-        DVsmApplyFrame::forward_children(obj<ACollector> gc) noexcept
+        void
+        DVsmApplyFrame::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.forward_inplace(&parent_);
-            gc.forward_inplace(&fn_);
-            gc.forward_inplace(&args_);
-
-            return this->shallow_size();
+            gc.visit_child(&parent_);
+            gc.visit_child(&fn_);
+            gc.visit_child(&args_);
         }
 
         bool
diff --git a/src/interpreter2/DVsmDefContFrame.cpp b/src/interpreter2/DVsmDefContFrame.cpp
index 02094534..0014f1a3 100644
--- a/src/interpreter2/DVsmDefContFrame.cpp
+++ b/src/interpreter2/DVsmDefContFrame.cpp
@@ -31,25 +31,17 @@ namespace xo {
 
         // gcobject facet
 
-        std::size_t
-        DVsmDefContFrame::shallow_size() const noexcept
-        {
-            return sizeof(*this);
-        }
-
         DVsmDefContFrame *
         DVsmDefContFrame::shallow_move(obj<ACollector> gc) noexcept
         {
             return gc.std_move_for<DVsmDefContFrame>(this);
         }
 
-        std::size_t
-        DVsmDefContFrame::forward_children(obj<ACollector> gc) noexcept
+        void
+        DVsmDefContFrame::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.forward_inplace(&parent_);
-            gc.forward_inplace(&def_expr_);
-
-            return this->shallow_size();
+            gc.visit_child(&parent_);
+            gc.visit_child(&def_expr_);
         }
 
         // printable facet
diff --git a/src/interpreter2/DVsmEvalArgsFrame.cpp b/src/interpreter2/DVsmEvalArgsFrame.cpp
index 204bfb44..b796bb78 100644
--- a/src/interpreter2/DVsmEvalArgsFrame.cpp
+++ b/src/interpreter2/DVsmEvalArgsFrame.cpp
@@ -41,25 +41,17 @@ namespace xo {
             return result;
         }
 
-        std::size_t
-        DVsmEvalArgsFrame::shallow_size() const noexcept
-        {
-            return sizeof(DVsmEvalArgsFrame);
-        }
-
         DVsmEvalArgsFrame *
         DVsmEvalArgsFrame::shallow_move(obj<ACollector> gc) noexcept
         {
             return gc.std_move_for(this);
         }
 
-        std::size_t
-        DVsmEvalArgsFrame::forward_children(obj<ACollector> gc) noexcept
+        void
+        DVsmEvalArgsFrame::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.forward_inplace(&parent_);
-            gc.forward_inplace(&apply_expr_);
-
-            return this->shallow_size();
+            gc.visit_child(&parent_);
+            gc.visit_child(&apply_expr_);
         }
 
         bool
diff --git a/src/interpreter2/DVsmIfElseContFrame.cpp b/src/interpreter2/DVsmIfElseContFrame.cpp
index 511af900..d0edd976 100644
--- a/src/interpreter2/DVsmIfElseContFrame.cpp
+++ b/src/interpreter2/DVsmIfElseContFrame.cpp
@@ -29,25 +29,17 @@ namespace xo {
 
         // gcobject facet
 
-        std::size_t
-        DVsmIfElseContFrame::shallow_size() const noexcept
-        {
-            return sizeof(*this);
-        }
-
         DVsmIfElseContFrame *
         DVsmIfElseContFrame::shallow_move(obj<ACollector> gc) noexcept
         {
             return gc.std_move_for<DVsmIfElseContFrame>(this);
         }
 
-        std::size_t
-        DVsmIfElseContFrame::forward_children(obj<ACollector> gc) noexcept
+        void
+        DVsmIfElseContFrame::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.forward_inplace(&parent_);
-            gc.forward_inplace(&ifelse_expr_);
-
-            return this->shallow_size();
+            gc.visit_child(&parent_);
+            gc.visit_child(&ifelse_expr_);
         }
 
         // printable facet
diff --git a/src/interpreter2/DVsmSeqContFrame.cpp b/src/interpreter2/DVsmSeqContFrame.cpp
index ad006def..f9ef2f0b 100644
--- a/src/interpreter2/DVsmSeqContFrame.cpp
+++ b/src/interpreter2/DVsmSeqContFrame.cpp
@@ -32,25 +32,17 @@ namespace xo {
 
         // gcobject facet
 
-        std::size_t
-        DVsmSeqContFrame::shallow_size() const noexcept
-        {
-            return sizeof(*this);
-        }
-
         DVsmSeqContFrame *
         DVsmSeqContFrame::shallow_move(obj<ACollector> gc) noexcept
         {
             return gc.std_move_for<DVsmSeqContFrame>(this);
         }
 
-        std::size_t
-        DVsmSeqContFrame::forward_children(obj<ACollector> gc) noexcept
+        void
+        DVsmSeqContFrame::visit_gco_children(obj<AGCObjectVisitor> gc) noexcept
         {
-            gc.forward_inplace(&parent_);
-            gc.forward_inplace(&seq_expr_);
-
-            return this->shallow_size();
+            gc.visit_child(&parent_);
+            gc.visit_child(&seq_expr_);
         }
 
         // printable facet
diff --git a/src/interpreter2/IGCObject_DClosure.cpp b/src/interpreter2/IGCObject_DClosure.cpp
index 3f6af457..889567bc 100644
--- a/src/interpreter2/IGCObject_DClosure.cpp
+++ b/src/interpreter2/IGCObject_DClosure.cpp
@@ -21,9 +21,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DClosure::forward_children(DClosure & self, obj<ACollector> gc) noexcept -> void
+        IGCObject_DClosure::visit_gco_children(DClosure & self, obj<AGCObjectVisitor> fn) noexcept -> void
         {
-            self.forward_children(gc);
+            self.visit_gco_children(fn);
         }
 
     } /*namespace scm*/
diff --git a/src/interpreter2/IGCObject_DVsmApplyClosureFrame.cpp b/src/interpreter2/IGCObject_DVsmApplyClosureFrame.cpp
index cd0bed84..be67179b 100644
--- a/src/interpreter2/IGCObject_DVsmApplyClosureFrame.cpp
+++ b/src/interpreter2/IGCObject_DVsmApplyClosureFrame.cpp
@@ -21,9 +21,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DVsmApplyClosureFrame::forward_children(DVsmApplyClosureFrame & self, obj<ACollector> gc) noexcept -> void
+        IGCObject_DVsmApplyClosureFrame::visit_gco_children(DVsmApplyClosureFrame & self, obj<AGCObjectVisitor> fn) noexcept -> void
         {
-            self.forward_children(gc);
+            self.visit_gco_children(fn);
         }
 
     } /*namespace scm*/
diff --git a/src/interpreter2/IGCObject_DVsmApplyFrame.cpp b/src/interpreter2/IGCObject_DVsmApplyFrame.cpp
index 0024df69..d3078821 100644
--- a/src/interpreter2/IGCObject_DVsmApplyFrame.cpp
+++ b/src/interpreter2/IGCObject_DVsmApplyFrame.cpp
@@ -21,9 +21,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DVsmApplyFrame::forward_children(DVsmApplyFrame & self, obj<ACollector> gc) noexcept -> void
+        IGCObject_DVsmApplyFrame::visit_gco_children(DVsmApplyFrame & self, obj<AGCObjectVisitor> fn) noexcept -> void
         {
-            self.forward_children(gc);
+            self.visit_gco_children(fn);
         }
 
     } /*namespace scm*/
diff --git a/src/interpreter2/IGCObject_DVsmDefContFrame.cpp b/src/interpreter2/IGCObject_DVsmDefContFrame.cpp
index e0cc2158..b3de6d94 100644
--- a/src/interpreter2/IGCObject_DVsmDefContFrame.cpp
+++ b/src/interpreter2/IGCObject_DVsmDefContFrame.cpp
@@ -21,9 +21,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DVsmDefContFrame::forward_children(DVsmDefContFrame & self, obj<ACollector> gc) noexcept -> void
+        IGCObject_DVsmDefContFrame::visit_gco_children(DVsmDefContFrame & self, obj<AGCObjectVisitor> fn) noexcept -> void
         {
-            self.forward_children(gc);
+            self.visit_gco_children(fn);
         }
 
     } /*namespace scm*/
diff --git a/src/interpreter2/IGCObject_DVsmEvalArgsFrame.cpp b/src/interpreter2/IGCObject_DVsmEvalArgsFrame.cpp
index 8107e7d9..edf1768b 100644
--- a/src/interpreter2/IGCObject_DVsmEvalArgsFrame.cpp
+++ b/src/interpreter2/IGCObject_DVsmEvalArgsFrame.cpp
@@ -21,9 +21,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DVsmEvalArgsFrame::forward_children(DVsmEvalArgsFrame & self, obj<ACollector> gc) noexcept -> void
+        IGCObject_DVsmEvalArgsFrame::visit_gco_children(DVsmEvalArgsFrame & self, obj<AGCObjectVisitor> fn) noexcept -> void
         {
-            self.forward_children(gc);
+            self.visit_gco_children(fn);
         }
 
     } /*namespace scm*/
diff --git a/src/interpreter2/IGCObject_DVsmIfElseContFrame.cpp b/src/interpreter2/IGCObject_DVsmIfElseContFrame.cpp
index 7b95a521..6f73f5a8 100644
--- a/src/interpreter2/IGCObject_DVsmIfElseContFrame.cpp
+++ b/src/interpreter2/IGCObject_DVsmIfElseContFrame.cpp
@@ -21,9 +21,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DVsmIfElseContFrame::forward_children(DVsmIfElseContFrame & self, obj<ACollector> gc) noexcept -> void
+        IGCObject_DVsmIfElseContFrame::visit_gco_children(DVsmIfElseContFrame & self, obj<AGCObjectVisitor> fn) noexcept -> void
         {
-            self.forward_children(gc);
+            self.visit_gco_children(fn);
         }
 
     } /*namespace scm*/
diff --git a/src/interpreter2/IGCObject_DVsmSeqContFrame.cpp b/src/interpreter2/IGCObject_DVsmSeqContFrame.cpp
index c9651507..437ac776 100644
--- a/src/interpreter2/IGCObject_DVsmSeqContFrame.cpp
+++ b/src/interpreter2/IGCObject_DVsmSeqContFrame.cpp
@@ -21,9 +21,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DVsmSeqContFrame::forward_children(DVsmSeqContFrame & self, obj<ACollector> gc) noexcept -> void
+        IGCObject_DVsmSeqContFrame::visit_gco_children(DVsmSeqContFrame & self, obj<AGCObjectVisitor> fn) noexcept -> void
         {
-            self.forward_children(gc);
+            self.visit_gco_children(fn);
         }
 
     } /*namespace scm*/
diff --git a/src/interpreter2/facet/IGCObject_DLocalEnv.cpp b/src/interpreter2/facet/IGCObject_DLocalEnv.cpp
index 25b84b4e..2b8e4e93 100644
--- a/src/interpreter2/facet/IGCObject_DLocalEnv.cpp
+++ b/src/interpreter2/facet/IGCObject_DLocalEnv.cpp
@@ -21,9 +21,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DLocalEnv::forward_children(DLocalEnv & self, obj<ACollector> gc) noexcept -> void
+        IGCObject_DLocalEnv::visit_gco_children(DLocalEnv & self, obj<AGCObjectVisitor> fn) noexcept -> void
         {
-            self.forward_children(gc);
+            self.visit_gco_children(fn);
         }
 
     } /*namespace scm*/
diff --git a/src/interpreter2/facet/IGCObject_DVirtualSchematikaMachine.cpp b/src/interpreter2/facet/IGCObject_DVirtualSchematikaMachine.cpp
index 8f071003..7fa74f97 100644
--- a/src/interpreter2/facet/IGCObject_DVirtualSchematikaMachine.cpp
+++ b/src/interpreter2/facet/IGCObject_DVirtualSchematikaMachine.cpp
@@ -21,9 +21,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DVirtualSchematikaMachine::forward_children(DVirtualSchematikaMachine & self, obj<ACollector> gc) noexcept -> void
+        IGCObject_DVirtualSchematikaMachine::visit_gco_children(DVirtualSchematikaMachine & self, obj<AGCObjectVisitor> fn) noexcept -> void
         {
-            self.forward_children(gc);
+            self.visit_gco_children(fn);
         }
 
     } /*namespace scm*/