# File lib/rational.rb, line 148
  def - (a)
    if a.kind_of?(Rational)
      num = @numerator * a.denominator
      num_a = a.numerator * @denominator
      Rational(num - num_a, @denominator*a.denominator)
    elsif a.kind_of?(Integer)
      self - Rational.new!(a, 1)
    elsif a.kind_of?(Float)
      Float(self) - a
    else
      x, y = a.coerce(self)
      x - y
    end
  end