pairOf :: Parser a -> Parser b -> Parser (a, b)
pairOf pa pb = parens $ do
  a <- pa
  comma
  b <- pb
  return (a, b)

-- expressionPair ::= (Expression, Expression)
pairOf :: Parser a -> Parser b -> Parser (a, b)
pairOf pa pb = parens $ do
  a <- pa
  comma
  b <- pb
  return (a, b)

-- expressionPair ::= (Expression, Expression)
pairOf pa pb = parens $ do
  a <- pa
  comma
  b <- pb
  return (a, b)

-- expressionPair ::= (Expression, Expression)