# File lib/graphviz.rb, line 639
  def <<( oNode )
    raise( ArgumentError, "Edge between root graph and node or cluster not allowed!" ) if self.pg.nil?

    if( oNode.class == Array ) 
      oNode.each do |no|
        self << no
      end
    else
      return GraphViz::commonGraph( oNode, self ).add_edge( self, oNode )
    end
  end