code: public LeftTupleSinkNode removeLast() {
          if ( this.lastNode == null ) {
              return null;
          }
          final LeftTupleSinkNode node = this.lastNode;
          this.lastNode = node.getPreviousLeftTupleSinkNode();
          node.setPreviousLeftTupleSinkNode( null );
          if ( this.lastNode != null ) {
              this.lastNode.setNextLeftTupleSinkNode( null );
          } else {
              this.firstNode = this.lastNode;
          }
          this.size--;
          return node;
      }
docs: Remove the last node from the list. The previous node then becomes the last node. If this is the last
  node then both first and last node references are set to null.
  
  @return
  The first <code>TupleSinkNode</code>.