code: protected void addToDownstream(BioPAXElement pe, Graph graph)
  	{
  		AbstractNode node = (AbstractNode) graph.getGraphObject(pe);
  
  		if (node != null)
  		{
  			EdgeL3 edge = new EdgeL3(this, node, graph);
  			edge.setTranscription(true);
  
  			node.getUpstreamNoInit().add(edge);
  			this.getDownstreamNoInit().add(edge);
  		}
  	}
docs: Binds the given PhysicalEntity to the downstream.
  @param pe PhysicalEntity to bind
  @param graph Owner graph