def multiple_of?(x)
      zero? ||
      self == x ||
      (num? && x.num? && self % x == 0) ||
      (formula?(:*) && (f == x || g == x))
    end
def multiple_of?(x)
      zero? ||
      self == x ||
      (num? && x.num? && self % x == 0) ||
      (formula?(:*) && (f == x || g == x))
    end
TODO: Cyclomatic complexity for combinable? is too high. [7/6]