[Haskell-cafe] Is (==) commutative?

Dear all, with respect to formal verification of Haskell code I was wondering whether (==) of the Eq class is intended to be commutative (for many classes such requirements are informally stated in their description, since Eq does not have such a statement, I'm asking here). Or are there any known cases where commutativity of (==) is violated (due to strictness issues)? cheers chris