code: private void afterSet(E element) {
          switch(_lastCall) {
              case NEXT:
                  _previousNode = _stack.getListNode(_lastId);
                  break;
              case PREVIOUS:
                  _nextNode = _stack.getListNode(_lastId);
                  break;
              default:
                  throw new IllegalStateException();
          }
      }
docs: helper function to be called after set() and setValue()