docs: def decimal_to_ratio(d):
      """Convert Decimal d to exact integer ratio (numerator, denominator).
      """
      sign, digits, exp = d.as_tuple()
      if exp in ('F', 'n', 'N'):  # INF, NAN, sNAN
          assert not d.is_finite()
          raise ValueError
      num = 0
      for digit in digits:
          num = num * 10 + digit
      if sign:
          num = -num
      den = 10 ** -exp
      return (num, den)