diff --git a/include/xo/expression2/DGlobalSymtab.hpp b/include/xo/expression2/DGlobalSymtab.hpp
index c6fcdcf1..782e48f7 100644
--- a/include/xo/expression2/DGlobalSymtab.hpp
+++ b/include/xo/expression2/DGlobalSymtab.hpp
@@ -44,7 +44,7 @@ namespace xo {
                           dp<repr_type> type_map, DArray * types);
 
             /** move constructor (needed because dp<> deletes copy ctor) **/
-            DGlobalSymtab(DGlobalSymtab && other) = default;
+            DGlobalSymtab(DGlobalSymtab && other);
 
             /** create instance.
              *  Use memory from @p fixed_mm for @ref map_.
diff --git a/include/xo/expression2/define/IGCObject_DDefineExpr.hpp b/include/xo/expression2/define/IGCObject_DDefineExpr.hpp
index b2c45805..e01e7be7 100644
--- a/include/xo/expression2/define/IGCObject_DDefineExpr.hpp
+++ b/include/xo/expression2/define/IGCObject_DDefineExpr.hpp
@@ -57,7 +57,7 @@ namespace xo {
             /** move instance using allocator **/
             static Opaque shallow_move(DDefineExpr & self, obj<ACollector> gc) noexcept;
             /** during GC: forward immdiate children **/
-            static size_type forward_children(DDefineExpr & self, obj<ACollector> gc) noexcept;
+            static void forward_children(DDefineExpr & self, obj<ACollector> gc) noexcept;
             ///@}
         };
 
diff --git a/include/xo/expression2/detail/IGCObject_DApplyExpr.hpp b/include/xo/expression2/detail/IGCObject_DApplyExpr.hpp
index 75eac694..b85665c8 100644
--- a/include/xo/expression2/detail/IGCObject_DApplyExpr.hpp
+++ b/include/xo/expression2/detail/IGCObject_DApplyExpr.hpp
@@ -57,7 +57,7 @@ namespace xo {
             /** move instance using allocator **/
             static Opaque shallow_move(DApplyExpr & self, obj<ACollector> gc) noexcept;
             /** during GC: forward immdiate children **/
-            static size_type forward_children(DApplyExpr & self, obj<ACollector> gc) noexcept;
+            static void forward_children(DApplyExpr & self, obj<ACollector> gc) noexcept;
             ///@}
         };
 
diff --git a/include/xo/expression2/detail/IGCObject_DConstant.hpp b/include/xo/expression2/detail/IGCObject_DConstant.hpp
index 9cb72026..91bf9638 100644
--- a/include/xo/expression2/detail/IGCObject_DConstant.hpp
+++ b/include/xo/expression2/detail/IGCObject_DConstant.hpp
@@ -57,7 +57,7 @@ namespace xo {
             /** move instance using allocator **/
             static Opaque shallow_move(DConstant & self, obj<ACollector> gc) noexcept;
             /** during GC: forward immdiate children **/
-            static size_type forward_children(DConstant & self, obj<ACollector> gc) noexcept;
+            static void forward_children(DConstant & self, obj<ACollector> gc) noexcept;
             ///@}
         };
 
diff --git a/include/xo/expression2/detail/IGCObject_DIfElseExpr.hpp b/include/xo/expression2/detail/IGCObject_DIfElseExpr.hpp
index e7d6fcfd..59aae40f 100644
--- a/include/xo/expression2/detail/IGCObject_DIfElseExpr.hpp
+++ b/include/xo/expression2/detail/IGCObject_DIfElseExpr.hpp
@@ -57,7 +57,7 @@ namespace xo {
             /** move instance using allocator **/
             static Opaque shallow_move(DIfElseExpr & self, obj<ACollector> gc) noexcept;
             /** during GC: forward immdiate children **/
-            static size_type forward_children(DIfElseExpr & self, obj<ACollector> gc) noexcept;
+            static void forward_children(DIfElseExpr & self, obj<ACollector> gc) noexcept;
             ///@}
         };
 
diff --git a/include/xo/expression2/detail/IGCObject_DLambdaExpr.hpp b/include/xo/expression2/detail/IGCObject_DLambdaExpr.hpp
index 6cf2cd6c..2351256d 100644
--- a/include/xo/expression2/detail/IGCObject_DLambdaExpr.hpp
+++ b/include/xo/expression2/detail/IGCObject_DLambdaExpr.hpp
@@ -57,7 +57,7 @@ namespace xo {
             /** move instance using allocator **/
             static Opaque shallow_move(DLambdaExpr & self, obj<ACollector> gc) noexcept;
             /** during GC: forward immdiate children **/
-            static size_type forward_children(DLambdaExpr & self, obj<ACollector> gc) noexcept;
+            static void forward_children(DLambdaExpr & self, obj<ACollector> gc) noexcept;
             ///@}
         };
 
diff --git a/include/xo/expression2/detail/IGCObject_DSequenceExpr.hpp b/include/xo/expression2/detail/IGCObject_DSequenceExpr.hpp
index 9a9b3f9d..c3fba848 100644
--- a/include/xo/expression2/detail/IGCObject_DSequenceExpr.hpp
+++ b/include/xo/expression2/detail/IGCObject_DSequenceExpr.hpp
@@ -57,7 +57,7 @@ namespace xo {
             /** move instance using allocator **/
             static Opaque shallow_move(DSequenceExpr & self, obj<ACollector> gc) noexcept;
             /** during GC: forward immdiate children **/
-            static size_type forward_children(DSequenceExpr & self, obj<ACollector> gc) noexcept;
+            static void forward_children(DSequenceExpr & self, obj<ACollector> gc) noexcept;
             ///@}
         };
 
diff --git a/include/xo/expression2/detail/IGCObject_DVarRef.hpp b/include/xo/expression2/detail/IGCObject_DVarRef.hpp
index 3ca237b6..97b0f3d6 100644
--- a/include/xo/expression2/detail/IGCObject_DVarRef.hpp
+++ b/include/xo/expression2/detail/IGCObject_DVarRef.hpp
@@ -57,7 +57,7 @@ namespace xo {
             /** move instance using allocator **/
             static Opaque shallow_move(DVarRef & self, obj<ACollector> gc) noexcept;
             /** during GC: forward immdiate children **/
-            static size_type forward_children(DVarRef & self, obj<ACollector> gc) noexcept;
+            static void forward_children(DVarRef & self, obj<ACollector> gc) noexcept;
             ///@}
         };
 
diff --git a/include/xo/expression2/symtab/IGCObject_DGlobalSymtab.hpp b/include/xo/expression2/symtab/IGCObject_DGlobalSymtab.hpp
index 43436ac8..5de0f0da 100644
--- a/include/xo/expression2/symtab/IGCObject_DGlobalSymtab.hpp
+++ b/include/xo/expression2/symtab/IGCObject_DGlobalSymtab.hpp
@@ -57,7 +57,7 @@ namespace xo {
             /** move instance using allocator **/
             static Opaque shallow_move(DGlobalSymtab & self, obj<ACollector> gc) noexcept;
             /** during GC: forward immdiate children **/
-            static size_type forward_children(DGlobalSymtab & self, obj<ACollector> gc) noexcept;
+            static void forward_children(DGlobalSymtab & self, obj<ACollector> gc) noexcept;
             ///@}
         };
 
diff --git a/include/xo/expression2/symtab/IGCObject_DLocalSymtab.hpp b/include/xo/expression2/symtab/IGCObject_DLocalSymtab.hpp
index c8b9cca1..6b9d81a1 100644
--- a/include/xo/expression2/symtab/IGCObject_DLocalSymtab.hpp
+++ b/include/xo/expression2/symtab/IGCObject_DLocalSymtab.hpp
@@ -57,7 +57,7 @@ namespace xo {
             /** move instance using allocator **/
             static Opaque shallow_move(DLocalSymtab & self, obj<ACollector> gc) noexcept;
             /** during GC: forward immdiate children **/
-            static size_type forward_children(DLocalSymtab & self, obj<ACollector> gc) noexcept;
+            static void forward_children(DLocalSymtab & self, obj<ACollector> gc) noexcept;
             ///@}
         };
 
diff --git a/include/xo/expression2/typename/IGCObject_DTypename.hpp b/include/xo/expression2/typename/IGCObject_DTypename.hpp
index d002a847..5e94f460 100644
--- a/include/xo/expression2/typename/IGCObject_DTypename.hpp
+++ b/include/xo/expression2/typename/IGCObject_DTypename.hpp
@@ -55,7 +55,7 @@ namespace xo {
             /** move instance using allocator **/
             static Opaque shallow_move(DTypename & self, obj<ACollector> gc) noexcept;
             /** during GC: forward immdiate children **/
-            static size_type forward_children(DTypename & self, obj<ACollector> gc) noexcept;
+            static void forward_children(DTypename & self, obj<ACollector> gc) noexcept;
             ///@}
         };
 
diff --git a/include/xo/expression2/variable/IGCObject_DVariable.hpp b/include/xo/expression2/variable/IGCObject_DVariable.hpp
index 5fd34c21..f7745ca9 100644
--- a/include/xo/expression2/variable/IGCObject_DVariable.hpp
+++ b/include/xo/expression2/variable/IGCObject_DVariable.hpp
@@ -57,7 +57,7 @@ namespace xo {
             /** move instance using allocator **/
             static Opaque shallow_move(DVariable & self, obj<ACollector> gc) noexcept;
             /** during GC: forward immdiate children **/
-            static size_type forward_children(DVariable & self, obj<ACollector> gc) noexcept;
+            static void forward_children(DVariable & self, obj<ACollector> gc) noexcept;
             ///@}
         };
 
diff --git a/src/expression2/DGlobalSymtab.cpp b/src/expression2/DGlobalSymtab.cpp
index 58275e54..c125fb98 100644
--- a/src/expression2/DGlobalSymtab.cpp
+++ b/src/expression2/DGlobalSymtab.cpp
@@ -29,6 +29,14 @@ namespace xo {
         {
         }
 
+        DGlobalSymtab::DGlobalSymtab(DGlobalSymtab && other)
+        : var_map_{std::move(other.var_map_)},
+          vars_{std::move(other.vars_)},
+          type_map_{std::move(other.type_map_)},
+          types_{std::move(other.types_)}
+        {
+        }
+
         DGlobalSymtab *
         DGlobalSymtab::_make(obj<AAllocator> mm,
                              obj<AAllocator> aux_mm,
diff --git a/src/expression2/IGCObject_DApplyExpr.cpp b/src/expression2/IGCObject_DApplyExpr.cpp
index 2a731c0b..d891b9de 100644
--- a/src/expression2/IGCObject_DApplyExpr.cpp
+++ b/src/expression2/IGCObject_DApplyExpr.cpp
@@ -27,9 +27,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DApplyExpr::forward_children(DApplyExpr & self, obj<ACollector> gc) noexcept -> size_type
+        IGCObject_DApplyExpr::forward_children(DApplyExpr & self, obj<ACollector> gc) noexcept -> void
         {
-            return self.forward_children(gc);
+            self.forward_children(gc);
         }
 
     } /*namespace scm*/
diff --git a/src/expression2/IGCObject_DConstant.cpp b/src/expression2/IGCObject_DConstant.cpp
index ae5a701f..256ba991 100644
--- a/src/expression2/IGCObject_DConstant.cpp
+++ b/src/expression2/IGCObject_DConstant.cpp
@@ -27,9 +27,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DConstant::forward_children(DConstant & self, obj<ACollector> gc) noexcept -> size_type
+        IGCObject_DConstant::forward_children(DConstant & self, obj<ACollector> gc) noexcept -> void
         {
-            return self.forward_children(gc);
+            self.forward_children(gc);
         }
 
     } /*namespace scm*/
diff --git a/src/expression2/IGCObject_DDefineExpr.cpp b/src/expression2/IGCObject_DDefineExpr.cpp
index 1e2c3bb8..b10d3ecd 100644
--- a/src/expression2/IGCObject_DDefineExpr.cpp
+++ b/src/expression2/IGCObject_DDefineExpr.cpp
@@ -27,9 +27,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DDefineExpr::forward_children(DDefineExpr & self, obj<ACollector> gc) noexcept -> size_type
+        IGCObject_DDefineExpr::forward_children(DDefineExpr & self, obj<ACollector> gc) noexcept -> void
         {
-            return self.forward_children(gc);
+            self.forward_children(gc);
         }
 
     } /*namespace scm*/
diff --git a/src/expression2/IGCObject_DGlobalSymtab.cpp b/src/expression2/IGCObject_DGlobalSymtab.cpp
index 3f596d01..70f541d3 100644
--- a/src/expression2/IGCObject_DGlobalSymtab.cpp
+++ b/src/expression2/IGCObject_DGlobalSymtab.cpp
@@ -27,9 +27,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DGlobalSymtab::forward_children(DGlobalSymtab & self, obj<ACollector> gc) noexcept -> size_type
+        IGCObject_DGlobalSymtab::forward_children(DGlobalSymtab & self, obj<ACollector> gc) noexcept -> void
         {
-            return self.forward_children(gc);
+            self.forward_children(gc);
         }
 
     } /*namespace scm*/
diff --git a/src/expression2/IGCObject_DIfElseExpr.cpp b/src/expression2/IGCObject_DIfElseExpr.cpp
index 240906f2..05e997cf 100644
--- a/src/expression2/IGCObject_DIfElseExpr.cpp
+++ b/src/expression2/IGCObject_DIfElseExpr.cpp
@@ -27,9 +27,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DIfElseExpr::forward_children(DIfElseExpr & self, obj<ACollector> gc) noexcept -> size_type
+        IGCObject_DIfElseExpr::forward_children(DIfElseExpr & self, obj<ACollector> gc) noexcept -> void
         {
-            return self.forward_children(gc);
+            self.forward_children(gc);
         }
 
     } /*namespace scm*/
diff --git a/src/expression2/IGCObject_DLambdaExpr.cpp b/src/expression2/IGCObject_DLambdaExpr.cpp
index c0a95847..0999c696 100644
--- a/src/expression2/IGCObject_DLambdaExpr.cpp
+++ b/src/expression2/IGCObject_DLambdaExpr.cpp
@@ -27,9 +27,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DLambdaExpr::forward_children(DLambdaExpr & self, obj<ACollector> gc) noexcept -> size_type
+        IGCObject_DLambdaExpr::forward_children(DLambdaExpr & self, obj<ACollector> gc) noexcept -> void
         {
-            return self.forward_children(gc);
+            self.forward_children(gc);
         }
 
     } /*namespace scm*/
diff --git a/src/expression2/IGCObject_DLocalSymtab.cpp b/src/expression2/IGCObject_DLocalSymtab.cpp
index 02941925..bf6d3b51 100644
--- a/src/expression2/IGCObject_DLocalSymtab.cpp
+++ b/src/expression2/IGCObject_DLocalSymtab.cpp
@@ -27,9 +27,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DLocalSymtab::forward_children(DLocalSymtab & self, obj<ACollector> gc) noexcept -> size_type
+        IGCObject_DLocalSymtab::forward_children(DLocalSymtab & self, obj<ACollector> gc) noexcept -> void
         {
-            return self.forward_children(gc);
+            self.forward_children(gc);
         }
 
     } /*namespace scm*/
diff --git a/src/expression2/IGCObject_DSequenceExpr.cpp b/src/expression2/IGCObject_DSequenceExpr.cpp
index 93af6f10..9ba1cab8 100644
--- a/src/expression2/IGCObject_DSequenceExpr.cpp
+++ b/src/expression2/IGCObject_DSequenceExpr.cpp
@@ -27,9 +27,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DSequenceExpr::forward_children(DSequenceExpr & self, obj<ACollector> gc) noexcept -> size_type
+        IGCObject_DSequenceExpr::forward_children(DSequenceExpr & self, obj<ACollector> gc) noexcept -> void
         {
-            return self.forward_children(gc);
+            self.forward_children(gc);
         }
 
     } /*namespace scm*/
diff --git a/src/expression2/IGCObject_DTypename.cpp b/src/expression2/IGCObject_DTypename.cpp
index 58df5bff..76bf837f 100644
--- a/src/expression2/IGCObject_DTypename.cpp
+++ b/src/expression2/IGCObject_DTypename.cpp
@@ -27,9 +27,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DTypename::forward_children(DTypename & self, obj<ACollector> gc) noexcept -> size_type
+        IGCObject_DTypename::forward_children(DTypename & self, obj<ACollector> gc) noexcept -> void
         {
-            return self.forward_children(gc);
+            self.forward_children(gc);
         }
 
     } /*namespace scm*/
diff --git a/src/expression2/IGCObject_DVarRef.cpp b/src/expression2/IGCObject_DVarRef.cpp
index 64967c9b..e9869fdf 100644
--- a/src/expression2/IGCObject_DVarRef.cpp
+++ b/src/expression2/IGCObject_DVarRef.cpp
@@ -27,9 +27,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DVarRef::forward_children(DVarRef & self, obj<ACollector> gc) noexcept -> size_type
+        IGCObject_DVarRef::forward_children(DVarRef & self, obj<ACollector> gc) noexcept -> void
         {
-            return self.forward_children(gc);
+            self.forward_children(gc);
         }
 
     } /*namespace scm*/
diff --git a/src/expression2/facet/IGCObject_DVariable.cpp b/src/expression2/facet/IGCObject_DVariable.cpp
index d418145c..657ce453 100644
--- a/src/expression2/facet/IGCObject_DVariable.cpp
+++ b/src/expression2/facet/IGCObject_DVariable.cpp
@@ -27,9 +27,9 @@ namespace xo {
             return self.shallow_move(gc);
         }
         auto
-        IGCObject_DVariable::forward_children(DVariable & self, obj<ACollector> gc) noexcept -> size_type
+        IGCObject_DVariable::forward_children(DVariable & self, obj<ACollector> gc) noexcept -> void
         {
-            return self.forward_children(gc);
+            self.forward_children(gc);
         }
 
     } /*namespace scm*/