code: func (f Index) PutInBatch(batch *leveldb.Batch, i Item) (err error) {
  	key, err := f.encodeKeyFunc(i)
  	if err != nil {
  		return err
  	}
  	value, err := f.encodeValueFunc(i)
  	if err != nil {
  		return err
  	}
  	batch.Put(key, value)
  	return nil
  }
docs: // PutInBatch is the same as Put method, but it just
  // saves the key/value pair to the batch instead
  // directly to the database.