docs: public T removeFirst() {
          if ( this.firstNode == null ) {
              return null;
          }
          final T node = this.firstNode;
          this.firstNode = node.getNext();
          node.setNext( null );
          if ( this.firstNode != null ) {
              this.firstNode.setPrevious( null );
          } else {
              this.lastNode = null;
          }
          this.size--;
          return node;
      }