Update: I've rewritten my solution above to be as close as possible to the original Arc code. All the function interfaces should be the same. Equality is determined with (=), which is the closest thing OCaml has to Arc's is.
I've also created a separate version that's closer to how I would prefer to implement it:
1. The comparison function is specified using a functor. This allows the user to specify it only once instead of in every call to insert, find, and so on. (I'd like to see something like ML's functors in Lisp someday.)
2. The comparison function handles equality as well as order.
3. edge is replaced by min_elt and max_elt.
4. rem-edge is gone.
5. The output of to_list (aka elts) isn't backwards.
