class RubyXL::Stylesheet

def register_number_format(format_code)
  self.number_formats ||= RubyXL::NumberFormats.new
  max_fmt_id = 163
  (NumberFormats::DEFAULT_NUMBER_FORMATS + number_formats).each { |fmt|
    return fmt.num_fmt_id if fmt.format_code == format_code
    max_fmt_id = fmt.num_fmt_id if fmt.num_fmt_id > max_fmt_id
  }
  max_fmt_id += 1
  number_formats << RubyXL::NumberFormat.new(:num_fmt_id => max_fmt_id, :format_code => format_code)
  return max_fmt_id
end