code: public void add(final LeftTupleSinkNode node) {
          if ( this.firstNode == null ) {
              this.firstNode = node;
              this.lastNode = node;;
          } else {
              this.lastNode.setNextLeftTupleSinkNode( node );
              node.setPreviousLeftTupleSinkNode( this.lastNode );
              this.lastNode = node;
          }
          this.size++;
      }
docs: Add a <code>TupleSinkNode</code> to the list. If the <code>LinkedList</code> is empty then the first and
  last nodes are set to the added node.
  
  @param node
  The <code>TupleSinkNode</code> to be added