parens :: Parser a -> Parser a
parens a = do
  _ <- char '('
  a' <- a
  _ <- char ')'
  return a'
parens :: Parser a -> Parser a
parens a = do
  _ <- char '('
  a' <- a
  _ <- char ')'
  return a'
parens a = do
  _ <- char '('
  a' <- a
  _ <- char ')'
  return a'