prop_strict_fold :: BagTest Int bag => bag Int -> bag Int -> Bool
prop_strict_fold bag xs =
    fold' (+) 0 xs == fold (+) 0 xs
    &&
    (null xs || fold1' (+) xs == fold1 (+) xs)
prop_strict_fold :: BagTest Int bag => bag Int -> bag Int -> Bool
prop_strict_fold bag xs =
    fold' (+) 0 xs == fold (+) 0 xs
    &&
    (null xs || fold1' (+) xs == fold1 (+) xs)
prop_strict_fold bag xs =
    fold' (+) 0 xs == fold (+) 0 xs
    &&
    (null xs || fold1' (+) xs == fold1 (+) xs)