abs' :: Int -> Int
abs' n | n>=0 = n
| otherwise = -n
signum' :: Int -> Int
signum' n | n > 0 = 1
| n < 0 = -1
| otherwise = 0
(&&&) :: Bool -> Bool -> Bool
True &&& True = True
_ &&& _ = False
head' :: [a] -> a
head' (x:_) = x
tail' :: [a] -> [a]
tail' (_:xs) = xs