def combine(operator, other)
      if other.respond_to?(:composition) && compatible_with?(other)
        new(value.send(operator, other.convert_to(unit).value), unit)
      end
    end
def combine(operator, other)
      if other.respond_to?(:composition) && compatible_with?(other)
        new(value.send(operator, other.convert_to(unit).value), unit)
      end
    end
Add or subtract other unit
 @api private