def convert_line(line_num)
      return '' unless @callouts.key? line_num

      @callouts[line_num]
        .map { |num| convert_callout(num) }
        .join(' ')
    end
def convert_line(line_num)
      return '' unless @callouts.key? line_num

      @callouts[line_num]
        .map { |num| convert_callout(num) }
        .join(' ')
    end
Converts the extracted callout markers for the specified line.

 @param line_num [Integer] the line number (1-based).
 @return [String] converted callout markers for the _line_num_,
   or an empty string if there are no callouts for that line.