Wren -- thanks for the clarification!  Someone said that Foldable on
Trie may not be very efficient -- is that true?

I use ByteString as a node type for the graph; these are Twitter user
names.  Surely it's useful to replace them with Int, which I'll try,
but Clojure works with Java String fine and it simplifies all kinds of
exploratory data mining and debugging to keep it as a String, so I'll
try to get the most mileage from other things before interning.

What's the exact relationship between Trie and Map and their
respective performance?