列出连接

listA      = [1, 2, 3]

listB      = [4, 5, 6]

listAThenB = listA ++ listB       -- [1, 2, 3, 4, 5, 6]

(++) xs     [] = xs
(++) []     ys = ys
(++) (x:xs) ys = x : (xs ++ ys)