Übung: merge

merge :: [ Int ] -> [ Int ] -> [ Int ]

-- merge [1,3,4,8] [2,5,6] 
--     = [1,2,3,4,5,6,8]

merge [] ys = ???
merge xs [] = ???
merge (x : xs) (y : ys) = 
    if x <= y then x : ???
              else ???