prop_unionSeq :: BagTest Int bag => bag Int -> Seq (bag Int) -> Bool
prop_unionSeq bag xss =
    unionSeq xss === S.foldr union empty xss
prop_unionSeq :: BagTest Int bag => bag Int -> Seq (bag Int) -> Bool
prop_unionSeq bag xss =
    unionSeq xss === S.foldr union empty xss
prop_unionSeq bag xss =
    unionSeq xss === S.foldr union empty xss