FUNCTION Flip(S: String) RETURN String IS
  -- Pre: S is defined
  -- Post: returns the reverse of S
 
  C : Character;  -- to save the first character of S
    
BEGIN -- Flip

  IF S'Length <= 1 THEN
    RETURN S;
  ELSE
    C := S(S'First);
    RETURN Flip(S(S'First + 1 .. S'Last)) & C;
  END IF;

END Flip;