diff --git a/xo-gc/utest/README.md b/xo-gc/utest/README.md
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+Unit test here relies on generated facet support.
+
+Also rehearsing subsystem setup that we use in other xo subsystems
+to perform facet registration.
+(This is for form's sake.  Could just as well invoke directly from gc_utest_main.cpp).
+
+idl/                    config for obj<AGCObjectVisitor,DMockCollector>.
+
+
+IGCObject_DMockCollector.*pp  automatically generated from idl/
+
+MockCollector.hpp       fop header file for DX1Collector.
+
+DMockCollector.*pp      mock ACollector impl. Wraps a GCObjectStore pointer.
+                        Load-bearing for GCObjectStore.test.cpp
+
+init_gc_utest.*pp       mock subsystem setup for this directory,
+                        as if it were an "xo-gc-utest" subsystem
+
+GCObjectStore.test.cpp  Generative tests for GCObjectStore.
+                        Generate non-trivial object graphs,
+                        verify evacuation algorithm.
+
+----------------------------------------------------------------
+These don't use anything downstream of idl/
+
+Collector.test.cpp      X1 tests. Does some random allocs,
+                        but doesn't exercise GC features
+
+X1Collector.test.cpp    standalone X1Collector test.
+                        does some atomic allocs + verifies
+                        GC behavior for a few xo-object2/ types.
+                        Verify simple tests here.
+
+DX1CollectorIterator.test.cpp  Collector iterator test