== Signature ==
  [] :: [Int]
 (:) :: Int -> [Int] -> [Int]
(++) :: [Int] -> [Int] -> [Int]
head :: [Int] -> Int
tail :: [Int] -> [Int]

== Laws ==
  1. xs ++ [] = xs
  2. [] ++ xs = xs
  3. head (x : xs) = x
  4. tail (x : xs) = xs
  5. (xs ++ ys) ++ zs = xs ++ (ys ++ zs)
  6. x : (xs ++ ys) = (x : xs) ++ ys