diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4dabb20..bb540b3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -41,7 +41,7 @@ xo_add_genfacetimpl(
 
 # must complete definition of expression lib before configuring examples
 add_subdirectory(src/object2)
-#add_subdirectory(utest)
+add_subdirectory(utest)
 #xo_export_cmake_config(${PROJECT_NAME} ${PROJECT_VERSION} ${PROJECT_NAME}Targets)
 
 # ----------------------------------------------------------------
diff --git a/include/xo/object2/DList.hpp b/include/xo/object2/DList.hpp
index 79cc84f..3f6a563 100644
--- a/include/xo/object2/DList.hpp
+++ b/include/xo/object2/DList.hpp
@@ -13,12 +13,24 @@ namespace xo {
         struct DList {
             using size_type = std::size_t;
             using AGCObject = xo::mm::AGCObject;
+            using AAllocator = xo::mm::AAllocator;
 
             DList(xo::obj<AGCObject> h,
                   DList * r) : head_{h}, rest_{r} {}
 
+            /** sentinel for null list **/
+            static DList * null();
+
+            /** list with one element @p h1, allocated from @p mm **/
+            static DList * list(obj<AAllocator> mm,
+                                obj<AGCObject> h1);
+            /** list with two elements @p h1, @p h2, allocated from @p mm **/
+            static DList * list(obj<AAllocator> mm,
+                                obj<AGCObject> h1,
+                                obj<AGCObject> h2);
+
             /** DList length is at least 1 **/
-            bool is_empty() const noexcept { return false; };
+            bool is_empty() const noexcept;
             /** DList models a finite sequence **/
             bool is_finite() const noexcept { return true; };
             /** return number of elements in this DList **/
@@ -26,7 +38,9 @@ namespace xo {
             /** return element at 0-based index @p ix **/
             obj<AGCObject> at(size_type ix) const;
 
+            /** first member of list **/
             obj<AGCObject> head_;
+            /** remainder of list **/
             DList * rest_ = nullptr;
         };
 
diff --git a/include/xo/object2/IGCObject_DFloat.hpp b/include/xo/object2/IGCObject_DFloat.hpp
index 3ccf40d..58b931b 100644
--- a/include/xo/object2/IGCObject_DFloat.hpp
+++ b/include/xo/object2/IGCObject_DFloat.hpp
@@ -5,13 +5,26 @@
 
 #pragma once
 
+#include <xo/alloc2/Allocator.hpp>
 #include <xo/gc/Collector.hpp>
-#include <xo/alloc2/alloc/AAllocator.hpp>
 #include <xo/gc/detail/AGCObject.hpp>
 #include <xo/gc/detail/IGCObject_Xfer.hpp>
 #include "DFloat.hpp"
 
 namespace xo {
+    namespace scm { struct IGCObject_DFloat; }
+
+    namespace facet {
+        template <>
+        struct FacetImplementation<xo::mm::AGCObject,
+                                   xo::scm::DFloat>
+        {
+            using ImplType = xo::mm::IGCObject_Xfer
+                <xo::scm::DFloat,
+                 xo::scm::IGCObject_DFloat>;
+        };
+    }
+
     namespace scm {
         /* changes here coordinate with:
          *  IGCObject_Xfer
@@ -19,11 +32,12 @@ namespace xo {
         struct IGCObject_DFloat {
         public:
             using AAllocator = xo::mm::AAllocator;
+            using ACollector = xo::mm::ACollector;
             using size_type = std::size_t;
 
             static size_type shallow_size(const DFloat & d) noexcept;
             static DFloat * shallow_copy(const DFloat & d, obj<AAllocator> mm) noexcept;
-            static size_type forward_children(DFloat & d) noexcept;
+            static size_type forward_children(DFloat & d, obj<ACollector> gc) noexcept;
         };
     } /*namespace scm*/
 } /*namespace xo*/
diff --git a/include/xo/object2/IGCObject_DInteger.hpp b/include/xo/object2/IGCObject_DInteger.hpp
index c53a79f..b306198 100644
--- a/include/xo/object2/IGCObject_DInteger.hpp
+++ b/include/xo/object2/IGCObject_DInteger.hpp
@@ -12,6 +12,19 @@
 #include "DInteger.hpp"
 
 namespace xo {
+    namespace scm { struct IGCObject_DInteger; }
+
+    namespace facet {
+        template <>
+        struct FacetImplementation<xo::mm::AGCObject,
+                                   xo::scm::DInteger>
+        {
+            using ImplType = xo::mm::IGCObject_Xfer
+                <xo::scm::DInteger,
+                 xo::scm::IGCObject_DInteger>;
+        };
+    }
+
     namespace scm {
         /* changes here coordinate with:
          *  IGCObject_Xfer
@@ -19,11 +32,12 @@ namespace xo {
         struct IGCObject_DInteger {
         public:
             using AAllocator = xo::mm::AAllocator;
+            using ACollector = xo::mm::ACollector;
             using size_type = std::size_t;
 
             static size_type shallow_size(const DInteger & d) noexcept;
             static DInteger * shallow_copy(const DInteger & d, obj<AAllocator> mm) noexcept;
-            static size_type forward_children(DInteger & d) noexcept;
+            static size_type forward_children(DInteger & d, obj<ACollector> gc) noexcept;
         };
     } /*namespace scm*/
 } /*namespace xo*/
diff --git a/include/xo/object2/IGCObject_DList.hpp b/include/xo/object2/IGCObject_DList.hpp
index c469d17..3d61a71 100644
--- a/include/xo/object2/IGCObject_DList.hpp
+++ b/include/xo/object2/IGCObject_DList.hpp
@@ -5,8 +5,7 @@
 
 #pragma once
 
-#include <xo/alloc2/alloc/AAllocator.hpp>
-#include <xo/alloc2/alloc/RAllocator.hpp>
+#include <xo/alloc2/Allocator.hpp>
 #include <xo/gc/Collector.hpp>
 #include <xo/gc/detail/AGCObject.hpp>
 #include <xo/gc/detail/IGCObject_Xfer.hpp>
diff --git a/include/xo/object2/object2_register_types.hpp b/include/xo/object2/object2_register_types.hpp
new file mode 100644
index 0000000..9b2656a
--- /dev/null
+++ b/include/xo/object2/object2_register_types.hpp
@@ -0,0 +1,19 @@
+/** @file object2_register_types.hpp
+ *
+ *  @author Roland Conybeare, Dec 2025
+ **/
+
+#pragma once
+
+#include <xo/gc/Collector.hpp>
+
+namespace xo {
+    namespace scm {
+        /** Register all object2/ gc-aware types with @p gc.
+         *  Return true iff all types register successfully.
+         **/
+        bool object2_register_types(obj<xo::mm::ACollector> gc);
+    }
+}
+
+/* end object2_register_types.hpp */
diff --git a/include/xo/object2/sequence/ISequence_Any.hpp b/include/xo/object2/sequence/ISequence_Any.hpp
index ebb172d..06838b3 100644
--- a/include/xo/object2/sequence/ISequence_Any.hpp
+++ b/include/xo/object2/sequence/ISequence_Any.hpp
@@ -14,6 +14,7 @@
 #pragma once
 
 #include "ASequence.hpp"
+#include <xo/facet/typeseq.hpp>
 #include <xo/facet/obj.hpp>
 
 namespace xo { namespace scm { class ISequence_Any; } }
@@ -42,6 +43,7 @@ namespace scm {
         /** @defgroup scm-sequence-any-type-traits **/
         ///@{
 
+        using typeseq = xo::facet::typeseq;
         using size_type = ASequence::size_type;
         using AGCObject = ASequence::AGCObject;
 
@@ -54,7 +56,7 @@ namespace scm {
         // from ASequence
 
         // const methods
-        int32_t _typeseq() const noexcept override { return s_typeseq; }
+        typeseq _typeseq() const noexcept override { return s_typeseq; }
         [[noreturn]] bool is_empty(Copaque) const noexcept override { _fatal(); }
         [[noreturn]] bool is_finite(Copaque) const noexcept override { _fatal(); }
         [[noreturn]] obj<AGCObject> at(Copaque, size_type) const override { _fatal(); }
@@ -75,7 +77,7 @@ namespace scm {
         /** @defgraoup scm-sequence-any-member-vars **/
         ///@{
 
-        static int32_t s_typeseq;
+        static typeseq s_typeseq;
         static bool _valid;
 
         ///@}
@@ -84,4 +86,4 @@ namespace scm {
 } /*namespace scm */
 } /*namespace xo */
 
-/* ISequence_Any.hpp */
\ No newline at end of file
+/* ISequence_Any.hpp */
diff --git a/src/object2/CMakeLists.txt b/src/object2/CMakeLists.txt
index dd76874..e2ffee2 100644
--- a/src/object2/CMakeLists.txt
+++ b/src/object2/CMakeLists.txt
@@ -8,6 +8,7 @@ set(SELF_SRCS
     ISequence_Any.cpp
     ISequence_DList.cpp
     DList.cpp
+    object2_register_types.cpp
     )
 
 xo_add_shared_library4(${SELF_LIB} ${PROJECT_NAME}Targets ${PROJECT_VERSION} 1 ${SELF_SRCS})
diff --git a/src/object2/DList.cpp b/src/object2/DList.cpp
index 2377d18..99469d1 100644
--- a/src/object2/DList.cpp
+++ b/src/object2/DList.cpp
@@ -7,7 +7,42 @@
 #include <xo/indentlog/print/tag.hpp>
 
 namespace xo {
+    using xo::mm::AGCObject;
+
     namespace scm {
+        static DList s_null(obj<AGCObject>(), nullptr);
+
+        DList *
+        DList::null()
+        {
+            return &s_null;
+        }
+
+        DList *
+        DList::list(obj<AAllocator> mm,
+                    obj<AGCObject> h1)
+        {
+            void * mem = mm.alloc(sizeof(DList));
+
+            return new (mem) DList(h1, DList::null());
+        }
+
+        DList *
+        DList::list(obj<AAllocator> mm,
+                    obj<AGCObject> h1,
+                    obj<AGCObject> h2)
+        {
+            void * mem = mm.alloc(sizeof(DList));
+
+            return new (mem) DList(h1, DList::list(mm, h2));
+        }
+
+        bool
+        DList::is_empty() const noexcept
+        {
+            return this != &s_null;
+        }
+
         auto
         DList::size() const noexcept -> size_type
         {
@@ -15,7 +50,7 @@ namespace xo {
 
             size_type z = 0;
 
-            while (l) {
+            while (l && l != &s_null) {
                 ++z;
                 l = l->rest_;
             }
diff --git a/src/object2/IGCObject_DFloat.cpp b/src/object2/IGCObject_DFloat.cpp
index 4c67631..a56175f 100644
--- a/src/object2/IGCObject_DFloat.cpp
+++ b/src/object2/IGCObject_DFloat.cpp
@@ -33,9 +33,10 @@ namespace xo {
         }
 
         size_t
-        IGCObject_DFloat::forward_children(DFloat &) noexcept
+        IGCObject_DFloat::forward_children(DFloat & src,
+                                           obj<ACollector>) noexcept
         {
-            return sizeof(DFloat);
+            return shallow_size(src);
         }
 
     } /*namespace scm*/
diff --git a/src/object2/IGCObject_DInteger.cpp b/src/object2/IGCObject_DInteger.cpp
index 0bd97f1..706ab09 100644
--- a/src/object2/IGCObject_DInteger.cpp
+++ b/src/object2/IGCObject_DInteger.cpp
@@ -33,9 +33,10 @@ namespace xo {
         }
 
         size_t
-        IGCObject_DInteger::forward_children(DInteger &) noexcept
+        IGCObject_DInteger::forward_children(DInteger & src,
+                                             obj<ACollector>) noexcept
         {
-            return sizeof(DInteger);
+            return shallow_size(src);
         }
 
     } /*namespace scm*/
diff --git a/src/object2/IGCObject_DList.cpp b/src/object2/IGCObject_DList.cpp
index d4355a2..19188df 100644
--- a/src/object2/IGCObject_DList.cpp
+++ b/src/object2/IGCObject_DList.cpp
@@ -37,8 +37,7 @@ namespace xo {
         {
             gc.forward_inplace(src.head_.iface(), (void **)&(src.head_.data_));
 
-            //auto rest = with_facet<AGCObject>::mkobj(src.rest_);
-            xo::facet::FacetImplementation<xo::mm::AGCObject, DList>::ImplType iface;
+            auto iface = xo::facet::impl_for<AGCObject, DList>();
             gc.forward_inplace(&iface, (void **)(&src.rest_));
 
             return shallow_size(src);
diff --git a/src/object2/object2_register_types.cpp b/src/object2/object2_register_types.cpp
new file mode 100644
index 0000000..e472401
--- /dev/null
+++ b/src/object2/object2_register_types.cpp
@@ -0,0 +1,34 @@
+/** @file object2_register_types.cpp
+ *
+ *  @author Roland Conybeare, Dec 2025
+ **/
+
+#include "object2_register_types.hpp"
+#include "IGCObject_DList.hpp"
+#include "IGCObject_DFloat.hpp"
+#include "IGCObject_DInteger.hpp"
+
+namespace xo {
+    using xo::mm::ACollector;
+    using xo::mm::AGCObject;
+    using xo::mm::IGCObject_Any;
+    using xo::facet::impl_for;
+    using xo::facet::typeseq;
+
+    namespace scm {
+
+        bool
+        object2_register_types(obj<ACollector> gc)
+        {
+            bool ok = true;
+
+            ok &= gc.install_type(impl_for<AGCObject, DList>());
+
+            ok &= gc.install_type(impl_for<AGCObject, DFloat>());
+
+            return ok;
+        }
+    }
+} /*namespace xo*/
+
+/* end object2_register_types.cpp */
diff --git a/utest/CMakeLists.txt b/utest/CMakeLists.txt
new file mode 100644
index 0000000..258f56a
--- /dev/null
+++ b/utest/CMakeLists.txt
@@ -0,0 +1,12 @@
+# built unittest xo-object2/utest
+
+set(UTEST_EXE utest.object2)
+set(UTEST_SRCS
+    object2_utest_main.cpp
+    X1Collector.test.cpp
+)
+
+xo_add_utest_executable(${UTEST_EXE} ${UTEST_SRCS})
+xo_self_dependency(${UTEST_EXE} xo_object2)
+#xo_dependency(${UTEST_EXE} randomgen)
+xo_external_target_dependency(${UTEST_EXE} Catch2 Catch2::Catch2)
diff --git a/utest/X1Collector.test.cpp b/utest/X1Collector.test.cpp
new file mode 100644
index 0000000..3857f0b
--- /dev/null
+++ b/utest/X1Collector.test.cpp
@@ -0,0 +1,117 @@
+/** @file X1Collector.test.cpp
+ *
+ *  @author Roland Conybeare, Dec 2025
+ **/
+
+#include <xo/gc/Collector.hpp>
+#include <xo/gc/DX1Collector.hpp>
+#include <catch2/catch.hpp>
+
+namespace ut {
+    using xo::mm::DX1Collector;
+    using xo::mm::DArena;
+    using xo::mm::CollectorConfig;
+    using xo::mm::ArenaConfig;
+    using xo::mm::generation;
+    using xo::mm::role;
+
+    namespace {
+        struct testcase_x1 {
+            testcase_x1(std::size_t nz,
+                        std::size_t tz,
+                        std::size_t n_gct,
+                        std::size_t t_gct)
+                : nursery_z_{nz},
+                  tenured_z_{tz},
+                  incr_gc_threshold_{n_gct},
+                  full_gc_threshold_{t_gct} {}
+
+            std::size_t nursery_z_;
+            std::size_t tenured_z_;
+            std::size_t incr_gc_threshold_;
+            std::size_t full_gc_threshold_;
+        };
+
+        std::vector<testcase_x1>
+        s_testcase_v = {
+            // n_gct: nursery gc threshold
+            // t_gct: tenured gc threshold
+            //
+            //            nz    tz  n_gct  t_gct
+            testcase_x1(4096, 8192,  1024,  1024)
+        };
+    }
+
+    TEST_CASE("x1", "[gc][x1]")
+    {
+        for (std::size_t i_tc = 0, n_tc = s_testcase_v.size(); i_tc < n_tc; ++i_tc) {
+            try {
+                const testcase_x1 & tc = s_testcase_v[i_tc];
+
+                CollectorConfig cfg{
+                    .name_ = "x1_test",
+                    .arena_config_ = ArenaConfig{
+                        .size_ = tc.tenured_z_,
+                        .store_header_flag_ = true},
+                    .object_types_z_ = 16384,
+                    .gc_trigger_v_{{
+                            tc.incr_gc_threshold_,
+                            tc.full_gc_threshold_}},
+                };
+
+                DX1Collector gc(cfg);
+
+                {
+                    REQUIRE(gc.name() == "x1_test");
+
+                    const DArena * otypes = gc.get_object_types();
+
+                    REQUIRE(otypes != nullptr);
+                    REQUIRE(otypes->reserved() >= cfg.object_types_z_);
+                    REQUIRE(otypes->reserved() < cfg.object_types_z_ + otypes->page_z_);
+
+                    DArena * from_0 = gc.get_space(role::from_space(), generation{0});
+
+                    REQUIRE(from_0 != nullptr);
+                    REQUIRE(from_0->reserved() >= tc.tenured_z_);
+                    REQUIRE(from_0->reserved() < tc.tenured_z_ + from_0->page_z_);
+                    REQUIRE(from_0->reserved() % from_0->page_z_ == 0);
+
+                    DArena * from_1 = gc.get_space(role::from_space(), generation{1});
+
+                    REQUIRE(from_1 != nullptr);
+                    REQUIRE(from_1->reserved() == from_0->reserved());
+
+                    DArena * to_0 = gc.get_space(role::to_space(), generation{0});
+
+                    REQUIRE(to_0 != nullptr);
+                    REQUIRE(to_0->reserved() == from_0->reserved());
+
+                    DArena * to_1 = gc.get_space(role::to_space(), generation{1});
+
+                    REQUIRE(to_1 != nullptr);
+                    REQUIRE(to_1->reserved() == to_0->reserved());
+
+                    DArena * from_2 = gc.get_space(role::from_space(), generation{2});
+
+                    REQUIRE(from_2 == nullptr);
+
+                    DArena * to_2 = gc.get_space(role::to_space(), generation{2});
+
+                    REQUIRE(to_2 == nullptr);
+
+                    REQUIRE(gc.reserved_total()
+                            == otypes->reserved() + 4 * from_0->reserved());
+                }
+
+                /* attempt allocation */
+
+            } catch (std::exception & ex) {
+                std::cerr << "caught exception: " << ex.what() << std::endl;
+                REQUIRE(false);
+            }
+        }
+    }
+}
+
+/* end X1Collector.test.cpp */
diff --git a/utest/object2_utest_main.cpp b/utest/object2_utest_main.cpp
new file mode 100644
index 0000000..a3dcb81
--- /dev/null
+++ b/utest/object2_utest_main.cpp
@@ -0,0 +1,6 @@
+/* file object2_utest_main.cpp */
+
+#define CATCH_CONFIG_MAIN
+#include "catch2/catch.hpp"
+
+/* end object2_utest_main.cpp */