3 The constraint `Eq a' does not mention any of the universally quantified type variables
4 in the type `(Eq a) => Set a -> Set a -> Set a'
5 In a type signature for `unionSets'
6 In an equation for function `unionSetB':
7 unionSetB (s1 :: Set a) s2
10 unionSets :: forall. (Eq a) => Set a -> Set a -> Set a