def approximate!(number_of_digits = nil)
      if number_of_digits.nil?
        if exact? && !repeating?
          @repeat = nil
        end
      else
        expand! number_of_digits
        @digits.truncate! number_of_digits
        @repeat = nil
      end
      self
    end
def approximate!(number_of_digits = nil)
      if number_of_digits.nil?
        if exact? && !repeating?
          @repeat = nil
        end
      else
        expand! number_of_digits
        @digits.truncate! number_of_digits
        @repeat = nil
      end
      self
    end
Expand to the specified number of digits,
 then truncate and remove repetitions.
 If no number of digits is given, then it will be
 converted to approximate numeral only if it is not
 repeating.