docs: public void set(Versioned<E> element) {
          if(element == null)
              throw new NullPointerException("cannot set a null element");
          if(_lastCall != LastCall.NEXT && _lastCall != LastCall.PREVIOUS)
              throw new IllegalStateException("neither next() nor previous() has been called");
  
          _stack.setById(_lastId, element);
  
          afterSet(element.getValue());
      }