+Note [Nodes, keys, vertices]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ * A 'node' is a big blob of client-stuff
+
+ * Each 'node' has a unique (client) 'key', but the latter
+ is in Ord and has fast comparison
+
+ * Digraph then maps each 'key' to a Vertex (Int) which is
+ arranged densely in 0.n
+