rnfail020.hs:12:
    The constraint `Eq a' does not mention any of the universally quantified type variables
	in the type `(Eq a) => Set a -> Set a -> Set a'
    In a type signature for `unionSets'
    In an equation for function `unionSetB':
	unionSetB (s1 :: Set a) s2
		    = unionSets s1 s2
		    where
			unionSets :: forall. (Eq a) => Set a -> Set a -> Set a
			unionSets a b = a