prop_fold :: BagTest Int bag => bag Int -> bag Int -> Bool
prop_fold bag xs =
    List.sort (fold (:) [] xs) == toOrdList xs
    &&
    (null xs || fold1 (+) xs == sum (toOrdList xs))
prop_fold :: BagTest Int bag => bag Int -> bag Int -> Bool
prop_fold bag xs =
    List.sort (fold (:) [] xs) == toOrdList xs
    &&
    (null xs || fold1 (+) xs == sum (toOrdList xs))
prop_fold bag xs =
    List.sort (fold (:) [] xs) == toOrdList xs
    &&
    (null xs || fold1 (+) xs == sum (toOrdList xs))