checkNull :: (s -> Bool) -> (s -> s -> Bool) -> (s -> s -> Bool)
checkNull null' comp s1 s2 = not (null' s1) && comp s1 s2
checkNull :: (s -> Bool) -> (s -> s -> Bool) -> (s -> s -> Bool)
checkNull null' comp s1 s2 = not (null' s1) && comp s1 s2
checkNull null' comp s1 s2 = not (null' s1) && comp s1 s2