mapMonotonic +containers

mapMonotonic :: (a -> b) -> Set a -> Set b
containers Data.Set
O(n). The mapMonotonic f s == map f s, but works only when f is monotonic. The precondition is not checked. Semi-formally, we have: > and [x < y ==> f x < f y | x <- ls, y <- ls] > ==> mapMonotonic f s == map f s >