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