diff --git a/idl/GCObject.json5 b/idl/GCObject.json5
index 4374758..0bbb170 100644
--- a/idl/GCObject.json5
+++ b/idl/GCObject.json5
@@ -1,24 +1,26 @@
 {
     mode: "facet",
-    output_cpp_dir: "src/gc",
-    output_hpp_dir: "include/xo/gc",
-    output_impl_subdir: "detail",
+    output_cpp_dir: "src/alloc2",
+    output_hpp_dir: "include/xo/alloc2",
+    output_impl_subdir: "gc",
     includes: [
         "<xo/alloc2/Allocator.hpp>",
-        "<xo/gc/Collector.hpp>",
+        "<xo/alloc2/Collector.hpp>",
         "<cstdint>",
         "<cstddef>",
     ],
     // extra includes in GCObject.hpp, if any
-    user_hpp_includes: [],
+    user_hpp_includes: [
+        "\"gc/RCollector_aux.hpp\"",
+    ],
     namespace1: "xo",
     namespace2: "mm",
     pretext: [
         "namespace xo { namespace mm { class ACollector; }}",
     ],
     facet: "GCObject",
-    detail_subdir: "detail",
-    brief: "xxx",
+    detail_subdir: "gc",
+    brief: "gc-aware object, providing collector hooks",
     using_doxygen: true,
     doc: [
         "GC hooks for collector-aware data"
diff --git a/include/xo/alloc2/GCObject.hpp b/include/xo/alloc2/GCObject.hpp
index b7758ac..4733b01 100644
--- a/include/xo/alloc2/GCObject.hpp
+++ b/include/xo/alloc2/GCObject.hpp
@@ -18,30 +18,6 @@
 #include "gc/IGCObject_Xfer.hpp"
 #include "gc/RGCObject.hpp"
 
-namespace xo {
-    namespace mm {
-        /** defined here to avoid #include cycle, since
-         *  template obj<AGCObject,DRepr> awkward to make available there
-         **/
-        template <typename Object>
-        template <typename DRepr>
-        void
-        RCollector<Object>::forward_inplace(xo::facet::obj<AGCObject,DRepr> * p_obj)
-        {
-            this->forward_inplace(p_obj->iface(), (void **)&(p_obj->data_));
-        }
-
-        template <typename Object>
-        template <typename DRepr>
-        void
-        RCollector<Object>::forward_inplace(DRepr ** p_repr)
-        {
-            // fetch static interface for DRepr
-            auto iface = xo::facet::impl_for<AGCObject,DRepr>();
-
-            this->forward_inplace(&iface, (void **)p_repr);
-        }
-    }
-}
+#include "gc/RCollector_aux.hpp"
 
 /* end GCObject.hpp */
diff --git a/include/xo/alloc2/GCObjectConversion.hpp b/include/xo/alloc2/GCObjectConversion.hpp
new file mode 100644
index 0000000..59ec12f
--- /dev/null
+++ b/include/xo/alloc2/GCObjectConversion.hpp
@@ -0,0 +1,117 @@
+/** @file GCObjectConversion.hpp
+ *
+ *  @author Roland Conybeare, Jan 2026
+ **/
+
+#pragma once
+
+#include <xo/alloc2/GCObject.hpp>
+#include <xo/alloc2/Allocator.hpp>
+#include <xo/facet/FacetRegistry.hpp>
+#include <xo/facet/obj.hpp>
+#include <xo/indentlog/scope.hpp>
+
+namespace xo {
+    namespace scm {
+        /** @brief compile-time conversion obj<AGCObject> <-> T
+         *
+         *  Specialize for each T that participates in conversion.
+         *  Methods here aren't implemented
+         **/
+        template <typename T>
+        struct GCObjectConversion {
+            using AGCObject = xo::mm::AGCObject;
+            using AAllocator = xo::mm::AAllocator;
+
+            /** find gc-aware representation for @p x.
+             *  If necessary allocate from @p mm, but may
+             *  refer to @p x in-place
+             **/
+            static obj<AGCObject> to_gco(obj<AAllocator> mm, const T & x);
+            /** convert to native representation @tparam T from gc-aware
+             *  @p gco.  If necessary allocate from @p mm, but
+             *  may instead refer to @p x in-place
+             **/
+            static T from_gco(obj<AAllocator> mm, obj<AGCObject> gco);
+        };
+
+        /** Motivating use-case for GCObjectConversion is to transform
+         *  primitive function arguments and results to/from gc-aware
+         *  representation.
+         *
+         *  However: Schematika also supports runtime polymorphism
+         *  which leads to primitives that expect obj<AFacet> arguments.
+         *
+         *  Also, Schematika expression parser needs representation for
+         *  expressions, before type unification.
+         *
+         *  Consider a function like:
+         *    def fact = lambda (n : i64) { if (n <= 0) then 1 else (n * fact(n - 1)); }
+         *  During expression parsing the rhs argument to multiply has unknown type.
+         *  To construct an expression for input to unification will use polymorphic
+         *  binding for multiply primitive, relying on specialization here for
+         *  its implementation.
+         **/
+        template <typename AFacet, typename DRepr>
+        struct GCObjectConversion<obj<AFacet,DRepr>> {
+            using AGCObject = xo::mm::AGCObject;
+            using AAllocator = xo::mm::AAllocator;
+            using FacetRegistry = xo::facet::FacetRegistry;
+            using DVariantPlaceholder = xo::facet::DVariantPlaceholder;
+
+            static obj<AGCObject> to_gco(obj<AAllocator>,
+                                         obj<AFacet,DRepr> gco) {
+                if constexpr (std::is_same_v<AFacet, AGCObject>) {
+                    // trivial conversion!
+                    return gco;
+                } else if constexpr (std::is_same_v<DRepr, DVariantPlaceholder>) {
+                    // runtime polymorphism
+                    return FacetRegistry::instance().variant<AGCObject,AFacet>(gco);
+                } else /* DRepr != DVariantPlaceholder */ {
+                    // known content w/ fat object pointer
+                    return obj<AGCObject,DRepr>(gco.data());
+                }
+            }
+
+            static obj<AFacet,DRepr> from_gco(obj<AAllocator>,
+                                              obj<AGCObject> gco) {
+                scope log(XO_DEBUG(false));
+
+                if constexpr (std::is_same_v<AFacet, AGCObject>) {
+                    // Need accurate handling of DVariantPlaceholder.
+                    // runtime type must be some concrete type.
+                    // Only use obj<AFacet,DRepr>::from when DRepr is a concrete type
+
+                    if constexpr (std::is_same_v<DRepr,DVariantPlaceholder>) {
+                        // At comptime gco has unknown repr.  At runtime
+                        // will have some known repr, which assignment here will transfer
+                        return gco;
+                    } else {
+                        // Runtime conversion to concrete type DRepr
+
+                        auto retval = obj<AFacet,DRepr>::from(gco);
+
+                        if (!retval) {
+                            log.retroactively_enable();
+
+                            log && log(xtag("gco.tseq", gco._typeseq()));
+                            log && log(xtag("DRepr.tseq", reflect::typeseq::id<DRepr>()));
+                        }
+
+                        assert(retval);
+
+                        return retval;
+                    }
+                } else {
+                    // both runtime and comptime polymorphism
+                    // use same path here, since representation of @p gco
+                    // is type-erased here
+
+                    return FacetRegistry::instance().variant<AFacet,AGCObject>(gco);
+                }
+            }
+        };
+    } /*namespace scm */
+} /*namespace xo*/
+
+/* end GCObjectConversion.hpp */
diff --git a/include/xo/alloc2/gc/AGCObject.hpp b/include/xo/alloc2/gc/AGCObject.hpp
index 87fc666..c420f0f 100644
--- a/include/xo/alloc2/gc/AGCObject.hpp
+++ b/include/xo/alloc2/gc/AGCObject.hpp
@@ -31,7 +31,7 @@ using Copaque = const void *;
 using Opaque = void *;
 
 /**
-   GC hooks for collector-aware data
+GC hooks for collector-aware data
 **/
 class AGCObject {
 public:
diff --git a/include/xo/alloc2/gc/RCollector_aux.hpp b/include/xo/alloc2/gc/RCollector_aux.hpp
new file mode 100644
index 0000000..b85c73b
--- /dev/null
+++ b/include/xo/alloc2/gc/RCollector_aux.hpp
@@ -0,0 +1,40 @@
+/** @file RCollector_aux.hpp
+ *
+ *  Out-of-line definitions for RCollector template methods
+ *  that depend on RGCObject (avoiding #include cycle in RCollector.hpp).
+ *
+ *  Included via user_hpp_includes in GCObject.json5.
+ *
+ *  @author Roland Conybeare
+ **/
+
+#pragma once
+
+namespace xo {
+    namespace mm {
+        /** defined here to avoid #include cycle, since
+         *  template obj<AGCObject,DRepr> awkward to make available
+         *  in RCollector.hpp
+         **/
+        template <typename Object>
+        template <typename DRepr>
+        void
+        RCollector<Object>::forward_inplace(xo::facet::obj<AGCObject,DRepr> * p_obj)
+        {
+            this->forward_inplace(p_obj->iface(), (void **)&(p_obj->data_));
+        }
+
+        template <typename Object>
+        template <typename DRepr>
+        void
+        RCollector<Object>::forward_inplace(DRepr ** p_repr)
+        {
+            // fetch static interface for DRepr
+            auto iface = xo::facet::impl_for<AGCObject,DRepr>();
+
+            this->forward_inplace(&iface, (void **)p_repr);
+        }
+    }
+}
+
+/* end RCollector_aux.hpp */
diff --git a/include/xo/gc/GCObject.hpp b/include/xo/gc/GCObject.hpp
deleted file mode 100644
index 759e937..0000000
--- a/include/xo/gc/GCObject.hpp
+++ /dev/null
@@ -1,22 +0,0 @@
-/** @file GCObject.hpp
- *
- *  Generated automagically from ingredients:
- *  1. code generator:
- *       [xo-facet/codegen/genfacet]
- *     arguments:
- *       --input [idl/GCObject.json5]
- *  2. jinja2 template for facet .hpp file:
- *       [facet.hpp.j2]
- *  3. idl for facet methods
- *       [idl/GCObject.json5]
- **/
-
-#pragma once
-
-#include "detail/AGCObject.hpp"
-#include "detail/IGCObject_Any.hpp"
-#include "detail/IGCObject_Xfer.hpp"
-#include "detail/RGCObject.hpp"
-
-
-/* end GCObject.hpp */
diff --git a/include/xo/gc/detail/AGCObject.hpp b/include/xo/gc/detail/AGCObject.hpp
deleted file mode 100644
index 6f6e7b9..0000000
--- a/include/xo/gc/detail/AGCObject.hpp
+++ /dev/null
@@ -1,87 +0,0 @@
-/** @file AGCObject.hpp
- *
- *  Generated automagically from ingredients:
- *  1. code generator:
- *       [xo-facet/codegen/genfacet]
- *     arguments:
- *       --input [idl/GCObject.json5]
- *  2. jinja2 template for abstract facet .hpp file:
- *       [abstract_facet.hpp.j2]
- *  3. idl for facet methods
- *       [idl/GCObject.json5]
- **/
-
-#pragma once
-
-// includes (via {facet_includes})
-#include <xo/alloc2/Allocator.hpp>
-#include <xo/gc/Collector.hpp>
-#include <cstdint>
-#include <cstddef>
-#include <xo/facet/obj.hpp>
-#include <xo/facet/facet_implementation.hpp>
-#include <xo/facet/typeseq.hpp>
-
-namespace xo { namespace mm { class ACollector; }}
-
-namespace xo {
-namespace mm {
-
-using Copaque = const void *;
-using Opaque = void *;
-
-/**
-GC hooks for collector-aware data
-**/
-class AGCObject {
-public:
-    /** @defgroup mm-gcobject-type-traits **/
-    ///@{
-    // types
-    /** integer identifying a type **/
-    using typeseq = xo::facet::typeseq;
-    using Copaque = const void *;
-    using Opaque = void *;
-    /** type for an amount of memory **/
-    using size_type = std::size_t;
-    /** fomo allocator type **/
-    using AAllocator = xo::mm::AAllocator;
-    /** fomo collector type **/
-    using ACollector = xo::mm::ACollector;
-    ///@}
-
-    /** @defgroup mm-gcobject-methods **/
-    ///@{
-    // const methods
-    /** RTTI: unique id# for actual runtime data representation **/
-    virtual typeseq _typeseq() const noexcept = 0;
-    /** destroy instance @p d; calls c++ dtor only for actual runtime type; does not recover memory **/
-    virtual void _drop(Opaque d) const noexcept = 0;
-    /** memory consumption for this instance **/
-    virtual size_type shallow_size(Copaque data)  const  noexcept = 0;
-    /** copy instance using allocator **/
-    virtual Opaque shallow_copy(Copaque data, obj<AAllocator> mm)  const  noexcept = 0;
-
-    // nonconst methods
-    /** during GC: forward immdiate children **/
-    virtual size_type forward_children(Opaque data, obj<ACollector> gc)  const  noexcept = 0;
-    ///@}
-}; /*AGCObject*/
-
-/** Implementation IGCObject_DRepr of AGCObject for state DRepr
- *  should provide a specialization:
- *
- *    template <>
- *    struct xo::facet::FacetImplementation<AGCObject, DRepr> {
- *        using Impltype = IGCObject_DRepr;
- *    };
- *
- *  then IGCObject_ImplType<DRepr> --> IGCObject_DRepr
- **/
-template <typename DRepr>
-using IGCObject_ImplType = xo::facet::FacetImplType<AGCObject, DRepr>;
-
-} /*namespace mm*/
-} /*namespace xo*/
-
-/* AGCObject.hpp */
diff --git a/include/xo/gc/detail/IGCObject_Any.hpp b/include/xo/gc/detail/IGCObject_Any.hpp
deleted file mode 100644
index 9ff7bb2..0000000
--- a/include/xo/gc/detail/IGCObject_Any.hpp
+++ /dev/null
@@ -1,93 +0,0 @@
-/** @file IGCObject_Any.hpp
- *
- *  Generated automagically from ingredients:
- *  1. code generator:
- *       [xo-facet/codegen/genfacet]
- *     arguments:
- *       --input [idl/GCObject.json5]
- *  2. jinja2 template for abstract facet .hpp file:
- *       [iface_facet_any.hpp.j2]
- *  3. idl for facet methods
- *       [idl/GCObject.json5]
- **/
-
-#pragma once
-
-#include "AGCObject.hpp"
-#include <xo/facet/obj.hpp>
-
-namespace xo { namespace mm { class IGCObject_Any; } }
-
-namespace xo {
-namespace facet {
-
-template <>
-struct FacetImplementation<xo::mm::AGCObject,
-                           DVariantPlaceholder>
-{
-    using ImplType = xo::mm::IGCObject_Any;
-};
-
-}
-}
-
-namespace xo {
-namespace mm {
-
-    /** @class IGCObject_Any
-     *  @brief AGCObject implementation for empty variant instance
-     **/
-    class IGCObject_Any : public AGCObject {
-    public:
-        /** @defgroup mm-gcobject-any-type-traits **/
-        ///@{
-
-        /** integer identifying a type **/
-        using typeseq = xo::facet::typeseq;
-        using size_type = AGCObject::size_type;
-        using AAllocator = AGCObject::AAllocator;
-        using ACollector = AGCObject::ACollector;
-
-        ///@}
-        /** @defgroup mm-gcobject-any-methods **/
-        ///@{
-
-        const AGCObject * iface() const { return std::launder(this); }
-
-        // from AGCObject
-
-        // builtin methods
-        typeseq _typeseq() const noexcept override { return s_typeseq; }
-        [[noreturn]] void _drop(Opaque) const noexcept override { _fatal(); }
-
-        // const methods
-        [[noreturn]] size_type shallow_size(Copaque)  const  noexcept override { _fatal(); }
-        [[noreturn]] Opaque shallow_copy(Copaque, obj<AAllocator>)  const  noexcept override { _fatal(); }
-
-        // nonconst methods
-        [[noreturn]] size_type forward_children(Opaque, obj<ACollector>)  const  noexcept override;
-
-        ///@}
-
-    private:
-        /** @defgraoup mm-gcobject-any-private-methods **/
-        ///@{
-
-        [[noreturn]] static void _fatal();
-
-        ///@}
-
-    public:
-        /** @defgroup mm-gcobject-any-member-vars **/
-        ///@{
-
-        static typeseq s_typeseq;
-        static bool _valid;
-
-        ///@}
-    };
-
-} /*namespace mm */
-} /*namespace xo */
-
-/* IGCObject_Any.hpp */
diff --git a/include/xo/gc/detail/IGCObject_Xfer.hpp b/include/xo/gc/detail/IGCObject_Xfer.hpp
deleted file mode 100644
index 7f094ab..0000000
--- a/include/xo/gc/detail/IGCObject_Xfer.hpp
+++ /dev/null
@@ -1,95 +0,0 @@
-/** @file IGCObject_Xfer.hpp
- *
- *  Generated automagically from ingredients:
- *  1. code generator:
- *       [xo-facet/codegen/genfacet]
- *     arguments:
- *       --input [idl/GCObject.json5]
- *  2. jinja2 template for abstract facet .hpp file:
- *       [iface_facet_any.hpp.j2]
- *  3. idl for facet methods
- *       [idl/GCObject.json5]
- **/
-
-#pragma once
-
-#include <xo/alloc2/Allocator.hpp>
-#include <xo/gc/Collector.hpp>
-#include <cstdint>
-#include <cstddef>
-
-namespace xo {
-namespace mm {
-    /** @class IGCObject_Xfer
-     **/
-    template <typename DRepr, typename IGCObject_DRepr>
-    class IGCObject_Xfer : public AGCObject {
-    public:
-        /** @defgroup mm-gcobject-xfer-type-traits **/
-        ///@{
-        /** actual implementation (not generated; often delegates to DRepr) **/
-        using Impl = IGCObject_DRepr;
-        /** integer identifying a type **/
-        using typeseq = AGCObject::typeseq;
-        using size_type = AGCObject::size_type;
-        using AAllocator = AGCObject::AAllocator;
-        using ACollector = AGCObject::ACollector;
-        ///@}
-
-        /** @defgroup mm-gcobject-xfer-methods **/
-        ///@{
-
-        static const DRepr & _dcast(Copaque d) { return *(const DRepr *)d; }
-        static DRepr & _dcast(Opaque d) { return *(DRepr *)d; }
-
-        // from AGCObject
-
-        // builtin methods
-        typeseq _typeseq() const noexcept override { return s_typeseq; }
-        void _drop(Opaque d) const noexcept override { _dcast(d).~DRepr(); }
-
-        // const methods
-        size_type shallow_size(Copaque data)  const  noexcept override {
-            return I::shallow_size(_dcast(data));
-        }
-        Opaque shallow_copy(Copaque data, obj<AAllocator> mm)  const  noexcept override {
-            return I::shallow_copy(_dcast(data), mm);
-        }
-
-        // non-const methods
-        size_type forward_children(Opaque data, obj<ACollector> gc)  const  noexcept override {
-            return I::forward_children(_dcast(data), gc);
-        }
-
-        ///@}
-
-    private:
-        using I = Impl;
-
-    public:
-        /** @defgroup mm-gcobject-xfer-member-vars **/
-        ///@{
-
-        /** typeseq for template parameter DRepr **/
-        static typeseq s_typeseq;
-        /** true iff satisfies facet implementation **/
-        static bool _valid;
-
-        ///@}
-    };
-
-    template <typename DRepr, typename IGCObject_DRepr>
-    xo::facet::typeseq
-    IGCObject_Xfer<DRepr, IGCObject_DRepr>::s_typeseq
-      = xo::facet::typeseq::id<DRepr>();
-
-    template <typename DRepr, typename IGCObject_DRepr>
-    bool
-    IGCObject_Xfer<DRepr, IGCObject_DRepr>::_valid
-      = xo::facet::valid_facet_implementation<AGCObject,
-                                              IGCObject_Xfer>();
-
-} /*namespace mm */
-} /*namespace xo*/
-
-/* end IGCObject_Xfer.hpp */
diff --git a/include/xo/gc/detail/RGCObject.hpp b/include/xo/gc/detail/RGCObject.hpp
deleted file mode 100644
index d24994a..0000000
--- a/include/xo/gc/detail/RGCObject.hpp
+++ /dev/null
@@ -1,93 +0,0 @@
-/** @file RGCObject.hpp
- *
- *  Generated automagically from ingredients:
- *  1. code generator:
- *       [xo-facet/codegen/genfacet]
- *     arguments:
- *       --input [idl/GCObject.json5]
- *  2. jinja2 template for abstract facet .hpp file:
- *       [iface_facet_any.hpp.j2]
- *  3. idl for facet methods
- *       [idl/GCObject.json5]
- **/
-
-#pragma once
-
-#include "AGCObject.hpp"
-
-namespace xo {
-namespace mm {
-
-/** @class RGCObject
- **/
-template <typename Object>
-class RGCObject : public Object {
-private:
-    using O = Object;
-
-public:
-    /** @defgroup mm-gcobject-router-type-traits **/
-    ///@{
-    using ObjectType = Object;
-    using DataPtr = Object::DataPtr;
-    using typeseq = xo::reflect::typeseq;
-    using size_type = AGCObject::size_type;
-    using AAllocator = AGCObject::AAllocator;
-    using ACollector = AGCObject::ACollector;
-    ///@}
-
-    /** @defgroup mm-gcobject-router-ctors **/
-    ///@{
-    RGCObject() {}
-    RGCObject(Object::DataPtr data) : Object{std::move(data)} {}
-    RGCObject(const AGCObject * iface, void * data)
-      requires std::is_same_v<typename Object::DataType, xo::facet::DVariantPlaceholder>
-      : Object(iface, data) {}
-
-    ///@}
-    /** @defgroup mm-gcobject-router-methods **/
-    ///@{
-
-    // explicit injected content
-
-    // builtin methods
-    typeseq _typeseq() const noexcept { return O::iface()->_typeseq(); }
-    void _drop() const noexcept { O::iface()->_drop(O::data()); }
-
-    // const methods
-    size_type shallow_size()  const  noexcept {
-        return O::iface()->shallow_size(O::data());
-    }
-    Opaque shallow_copy(obj<AAllocator> mm)  const  noexcept {
-        return O::iface()->shallow_copy(O::data(), mm);
-    }
-
-    // non-const methods (still const in router!)
-    size_type forward_children(obj<ACollector> gc)  noexcept {
-        return O::iface()->forward_children(O::data(), gc);
-    }
-
-    ///@}
-    /** @defgroup mm-gcobject-member-vars **/
-    ///@{
-
-    static bool _valid;
-
-    ///@}
-};
-
-template <typename Object>
-bool
-RGCObject<Object>::_valid = xo::facet::valid_object_router<Object>();
-
-} /*namespace mm*/
-} /*namespace xo*/
-
-namespace xo { namespace facet {
-    template <typename Object>
-    struct RoutingFor<xo::mm::AGCObject, Object> {
-        using RoutingType = xo::mm::RGCObject<Object>;
-    };
-} }
-
-/* end RGCObject.hpp */