I think that this is one example where the types don't speak completely for themselves. Is `Neg Zero` meant to stand for the integer `0` (as one might naïvely expect) or the integer `-1` (as one might expect from the pseudocode `Neg n = -(n + 1)`)? If the former, then what is `subtract Zero Zero`? (I.e., is it `Zero` or `Neg Zero`? I hope that we don't get into IEEE 754 (http://en.wikipedia.org/wiki/Signed_zero) territory here!) If the latter, then there's no ambiguity, but I think that it's fair to say that that meaning isn't clear just from the type!