[Haskell-cafe] zigzag data structure

[Cale Gibbard]

> Essentially, the structure that they define on that site is a graph
> with arcs labelled in such a way that any vertex has at most one
> outgoing arc with that label (and hence at most one incoming arc with
> that label as well).

Sorry, I obviously didn't read that carefully enough, and both
invariants need enforcing. That is, you have to ensure that there is
at most one incoming arc with a given label as well.

 - Cale