23 Dec
2005
23 Dec
'05
4:21 a.m.
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