ch4_001.hs

safetail :: [a] -> [a]
safetail xs = if null xs then [] else tail xs

safetail' xs 
  | null xs = []
  | otherwise = tail xs 

safetail'' [] = []
safetail'' xs = tail xs

(||) :: Bool -> Bool -> Bool
(||) False False = False
(||) _ _ = True

(&&):: Bool -> Bool -> Bool
--(&&) x y = if x then if y then True else False else False
(&&) x y = if x then y else False