prop_strict_foldr_foldl :: BagTest Int bag => bag Int -> bag Int -> Bool
prop_strict_foldr_foldl bag xs =
    foldr' (+) 0 xs == foldr (+) 0 xs
    &&
    foldl' (+) 0 xs == foldl (+) 0 xs
prop_strict_foldr_foldl :: BagTest Int bag => bag Int -> bag Int -> Bool
prop_strict_foldr_foldl bag xs =
    foldr' (+) 0 xs == foldr (+) 0 xs
    &&
    foldl' (+) 0 xs == foldl (+) 0 xs
prop_strict_foldr_foldl bag xs =
    foldr' (+) 0 xs == foldr (+) 0 xs
    &&
    foldl' (+) 0 xs == foldl (+) 0 xs