code: func (iter *UnionIter) Next() error {
  	var err error
  	if !iter.curIsDirty {
  		err = iter.snapshotNext()
  	} else {
  		err = iter.dirtyNext()
  	}
  	if err != nil {
  		return err
  	}
  	err = iter.updateCur()
  	return err
  }
docs: // Next implements the Iterator Next interface.