[Haskell-cafe] Trying to Express Constraints using a data structure

Hi all, I'm trying to express a constraint using a data structure. Let's say I'd want to express a "mapping" of a to b, c to b, d to b and e to f. A mapping can also be from a to a, b to b and so on. The constraint is that one cannot map a to b if b was already "mapped" to let's say c. I'd like to express this constraint in a data structure, but haven't figured out how, yet. Any suggestions? Günther