code: func dbStoreFilterIdxEntry(dbTx database.Tx, key []byte, h *chainhash.Hash, f []byte) error {
  	idx := dbTx.Metadata().Bucket(cfIndexParentBucketKey).Bucket(key)
  	return idx.Put(h[:], f)
  }
docs: // dbStoreFilterIdxEntry stores a data blob in the filter index database.