diff --git a/xo-alloc/docs/glossary.rst b/xo-alloc/docs/glossary.rst
new file mode 100644
index 00000000..4e50a499
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+++ b/xo-alloc/docs/glossary.rst
@@ -0,0 +1,28 @@
+.. _glossary:
+
+Glossary
+--------
+
+.. glossary::
+    GC
+     | garbage collector
+
+    mlog
+     | mutation log.
+     | Remembers cross-generation and cross-checkpoint pointers
+
+    nursery
+     | in garbage collector, memory region dedicated to young objects.
+     | These are objects that have survived less than 2 incremental collection cycles.
+
+    tenured
+     | in garbage collector, memory region dedicated to older objects.
+     | These are defined as objects that have survived 2 or more incremental collection cycles.
+
+    xgen
+     | cross-generation tenured->nursery pointer; requires special GC bookkeeping
+
+    xckp
+     | cross-checkpoint pointer; requires special GC bookkeeping
+
+.. toctree::
diff --git a/xo-object/utest/GC.test.cpp b/xo-object/utest/GC.test.cpp
new file mode 100644
index 00000000..544dd149
--- /dev/null
+++ b/xo-object/utest/GC.test.cpp
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+/* @file GC.test.cpp
+ *
+ * author: Roland Conybeare, Aug 2025
+ */
+
+#include "xo/alloc/GC.hpp"
+#include "xo/object/List.hpp"
+#include "xo/object/Integer.hpp"
+#include <catch2/catch.hpp>
+
+namespace xo {
+    using xo::obj::List;
+    using xo::obj::Integer;
+    using xo::gc::GC;
+    using xo::gc::generation_result;
+    using xo::gc::generation;
+
+    namespace ut {
+
+        // Also see GC unit tests in xo-alloc/utest
+
+#ifdef NOT_YET
+        namespace {
+            struct testcase_mlog {
+                testcase_mlog(std::size_t nz, std::size_t tz) : nursery_z_{nz}, tenured_z_{tz} {}
+
+                std::size_t nursery_z_;
+                std::size_t tenured_z_;
+            };
+        }
+#endif
+
+        TEST_CASE("gc-mlog-1", "[alloc][gc][gc_mutation]")
+        {
+            up<GC> gc = GC::make(
+                {
+                    .initial_nursery_z_ = 1024,
+                    .initial_tenured_z_ = 2048,
+                    .debug_flag_ = true
+                });
+
+            REQUIRE(gc->gc_statistics().n_mutation_ == 0);
+            REQUIRE(gc->gc_statistics().n_logged_mutation_ == 0);
+
+            REQUIRE(gc.get());
+
+            /* use gc for all Object allocs */
+            Object::mm = gc.get();
+
+            gp<List> l = List::list(Integer::make(1));
+            gc->add_gc_root(reinterpret_cast<Object**>(l.ptr_address()));
+            {
+                REQUIRE(l->size() == 1);
+
+                REQUIRE(gc->tospace_generation_of(l.ptr()) == generation_result::nursery);
+                REQUIRE(gc->tospace_generation_of(l->head().ptr()) == generation_result::nursery);
+
+                REQUIRE(gc->is_before_checkpoint(l.ptr()) == false);
+                REQUIRE(gc->is_before_checkpoint(l->head().ptr()) == false);
+
+                REQUIRE(gc->mlog_size() == 0);
+            }
+
+            // mutation, but not {xgen, xckp} since parent,child both in N0
+
+            l->assign_head(Integer::make(2));
+            {
+                REQUIRE(gc->gc_statistics().n_mutation_ == 1);
+                REQUIRE(gc->gc_statistics().n_logged_mutation_ == 0);
+                REQUIRE(gc->gc_statistics().n_xgen_mutation_ == 0);
+                REQUIRE(gc->gc_statistics().n_xckp_mutation_ == 0);
+                REQUIRE(gc->mlog_size() == 0);
+
+                REQUIRE(gc->is_gc_enabled() == true);
+
+            }
+
+            gc->request_gc(generation::nursery);
+            {
+                REQUIRE(gc->is_before_checkpoint(l.ptr()) == true);
+                REQUIRE(gc->is_before_checkpoint(l->head().ptr()) == true);
+
+                REQUIRE(gc->tospace_generation_of(l.ptr()) == generation_result::nursery);
+
+                REQUIRE(l->size() == 1);
+                REQUIRE(Integer::from(l->head()).ptr());
+                REQUIRE(Integer::from(l->head())->value() == 2);
+            }
+
+            // mutation, xckp since parent in N1, child in N0
+
+            l->assign_head(Integer::make(3));
+            {
+                REQUIRE(Integer::from(l->head())->value() == 3);
+
+                REQUIRE(gc->tospace_generation_of(l->head().ptr()) == generation_result::nursery);
+                REQUIRE(gc->is_before_checkpoint(l->head().ptr()) == false);
+
+                REQUIRE(gc->gc_statistics().n_mutation_ == 2);
+                REQUIRE(gc->gc_statistics().n_logged_mutation_ == 1);
+                REQUIRE(gc->gc_statistics().n_xgen_mutation_ == 0);
+                REQUIRE(gc->gc_statistics().n_xckp_mutation_ == 1);
+                REQUIRE(gc->mlog_size() == 1);
+            }
+
+            // gc promotes parent, still need mutation log for xgen ptr
+
+            gc->request_gc(generation::nursery);
+            {
+                REQUIRE(l->size() == 1);
+                REQUIRE(Integer::from(l->head()).ptr());
+                REQUIRE(Integer::from(l->head())->value() == 3);
+
+                REQUIRE(gc->tospace_generation_of(l.ptr()) == generation_result::tenured);
+                REQUIRE(gc->tospace_generation_of(l->head().ptr()) == generation_result::nursery);
+                REQUIRE(gc->is_before_checkpoint(l->head().ptr()));
+
+                REQUIRE(gc->gc_statistics().n_mutation_ == 2);
+                REQUIRE(gc->gc_statistics().n_logged_mutation_ == 1);
+                // counters recorded when mutation created.
+                // not modified by gc
+                REQUIRE(gc->gc_statistics().n_xgen_mutation_ == 0);
+                REQUIRE(gc->gc_statistics().n_xckp_mutation_ == 1);
+                REQUIRE(gc->mlog_size() == 1);
+            }
+
+            // gc promotes child, no longer need mutation log entry
+
+            gc->request_gc(generation::nursery);
+            {
+                REQUIRE(l->size() == 1);
+                REQUIRE(Integer::from(l->head()).ptr());
+                REQUIRE(Integer::from(l->head())->value() == 3);
+
+                REQUIRE(gc->tospace_generation_of(l.ptr()) == generation_result::tenured);
+                REQUIRE(gc->tospace_generation_of(l->head().ptr()) == generation_result::tenured);
+
+                REQUIRE(gc->mlog_size() == 0);
+            }
+        }
+    } /*namespace ut*/
+} /*namespace xo*/
+
+/* end GC.test.cpp */