expr :: Parser Term
expr = chainl1 expr' f where
  f = do
    spaces
    char '+'
    spaces
    return $ BinOp Plus
expr :: Parser Term
expr = chainl1 expr' f where
  f = do
    spaces
    char '+'
    spaces
    return $ BinOp Plus
expr = chainl1 expr' f where
  f = do
    spaces
    char '+'
    spaces
    return $ BinOp Plus