code: public void setValue(E element) {
          if(element == null)
              throw new NullPointerException("cannot set a null element");
          if(_lastId == VStack.NULL_ID)
              throw new IllegalStateException("neither next() nor previous() has been called");
  
          _stack.setById(_lastId, element);
  
          afterSet(element);
      }
docs: /*
  Set the value of the last element returned by next or previous,
  discarding any version information