# File lib/rdoc/markup/document.rb, line 30
  def << part
    case part
    when RDoc::Markup::Document then
      unless part.empty? then
        parts.push(*part.parts)
        parts << RDoc::Markup::BlankLine.new
      end
    when String then
      raise ArgumentError,
            "expected RDoc::Markup::Document and friends, got String" unless
        part.empty?
    else
      parts << part
    end
  end