code: func NextUntil(it Iterator, fn FnKeyCmp) error {
  	var err error
  	for it.Valid() && !fn(it.Key()) {
  		err = it.Next()
  		if err != nil {
  			return err
  		}
  	}
  	return nil
  }
docs: // NextUntil applies FnKeyCmp to each entry of the iterator until meets some condition.
  // It will stop when fn returns true, or iterator is invalid or an error occurs.