code: @Override
  	public void accept(Visitor<PlanNode> visitor) {
  		if (visitor.preVisit(this)) {
  			this.input1.getSource().accept(visitor);
  			this.input2.getSource().accept(visitor);
  			
  			for (Channel broadcastInput : getBroadcastInputs()) {
  				broadcastInput.getSource().accept(visitor);
  			}
  			
  			visitor.postVisit(this);
  		}
  	}
docs: --------------------------------------------------------------------------------------------