class RDoc::Markup::Document

def merge other
  if empty? then
    @parts = other.parts
    return self
  end
  other.parts.each do |other_part|
    self.parts.delete_if do |self_part|
      self_part.file and self_part.file == other_part.file
    end
    self.parts << other_part
  end
  self
end