-- (append xs ys) returns the lists xs and ys appended
-- For example:
--   append [1,2,3] [5,2] = [1,2,3,5,2]

append            :: [a] -> [a] -> [a]
append [] ys      = ys
append            :: [a] -> [a] -> [a]
append [] ys      = ys
append [] ys      = ys