prop_strict_foldr1_foldl1 :: BagTest Int bag => bag Int -> bag Int -> Property
prop_strict_foldr1_foldl1 bag xs =
    not (null xs) ==>
       foldr1' (+) xs == foldr1 (+) xs
       &&
       foldl1' (+) xs == foldl1 (+) xs
prop_strict_foldr1_foldl1 :: BagTest Int bag => bag Int -> bag Int -> Property
prop_strict_foldr1_foldl1 bag xs =
    not (null xs) ==>
       foldr1' (+) xs == foldr1 (+) xs
       &&
       foldl1' (+) xs == foldl1 (+) xs
prop_strict_foldr1_foldl1 bag xs =
    not (null xs) ==>
       foldr1' (+) xs == foldr1 (+) xs
       &&
       foldl1' (+) xs == foldl1 (+) xs