You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: document.rb
    path of file: ./repos/ruby-packer/ruby/lib/rdoc/markup
    
    the code of the file until where you have to start completion: # frozen_string_literal: tru
    
  def << part
    case part
    when RDoc::Markup::Document then
      unless part.empty? then
        parts.concat part.parts
        parts << RDoc::Markup::BlankLine.new
      end