3 All of the type variables in the constraint `Eq a' are already in scope
4 At least one must be universally quantified here
5 In the type `(Eq a) => Set a -> Set a -> Set a'
6 In a type signature for `unionSets'
7 In the definition of `unionSetB':
8 unionSetB (s1 :: Set a) s2
11 unionSets :: forall. (Eq a) => Set a -> Set a -> Set a