kind=defined
names=+
visibility=public

--- +(other)

和を計算します。

  Rational(3, 4) + 2               # => Rational(11, 4)
  Rational(3, 4) + Rational(2, 1)  # => Rational(11, 4)
  Rational(3, 4) + 2.0             # => 2.75

