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+There still an issue with inferring for a given function call /
+constant reference where that thing is defined.
+
+We're currently generating the forest of references from the call
+graph by calculating a spanning forest.  There's a problem with that.
+Consider the following map of call-sites:
+
+    {
+      a: [b]
+      b: []
+    }
+
+The following is a spanning tree:
+
+    {
+      a: []
+      b: []
+    }
+
+Here it looks like `a` doesn't call `b`.
+
+
+We should also store all the call-sites.  These will end up as leaves
+in the call tree.
+
+
+    {
+      "K": {},
+      "A": {},
+      "f": {
+        "f": "__cycle__"
+      },
+      "A.f": {
+        "K.g": {}
+      }
+    }
+
+
+Write graphviz output.