main :: [sys_message]
main = [Stdout (show (symdiff a b) ++ "\n")]
       where a = ["John", "Serena", "Bob", "Mary", "Serena"]
             b = ["Jim", "Mary", "John", "Jim", "Bob"]

symdiff :: [*]->[*]->[*]
symdiff a b = (a' -- b') ++ (b' -- a')
              where a' = nub a
                    b' = nub b

nub :: [*]->[*]
nub = f []
      where f acc []     = acc
            f acc (a:as) = f acc as,     if a $in acc
                         = f (a:acc) as, otherwise

in :: *->[*]->bool
in i []     = False
in i (a:as) = a == i \/ i $in as