{- A multiline comment which can continue for many lines -} 

data Expr = Val Int | Div Expr Expr 
-- data Maybe a = Nothing | Just a


eval :: Expr -> Maybe Int
eval (Val n) = Just n
eval (Div x y) = case (eval x) of 
					Nothing -> Nothing
					Just n  -> case eval y of
						Nothing -> Nothing
						Just m -> safediv m n
						                    
					
safediv :: Int -> Int -> Maybe Int
safediv n m = if m == 0 then Nothing else Just (n `div` m)