code: func (bk *Backend) UpsertItems(bucket []string, newItems []legacy.Item) error {
  	// Open the bucket to work on the items.
  	b, err := bk.openBucket(bk.flatten(bucket), os.O_CREATE|os.O_RDWR)
  	if err != nil {
  		return trace.Wrap(err)
  	}
  	defer b.Close()
  
  	// Update items in bucket.
  	for _, e := range newItems {
  		b.updateItem(e.Key, e.Value, e.TTL)
  	}
  
  	return nil
  }
docs: // UpsertItems inserts (or updates if it already exists) all passed in
  // legacy.Items with the given TTL.