code: func (db *Database) secureKey(key []byte) []byte {
  	buf := append(db.seckeybuf[:0], secureKeyPrefix...)
  	buf = append(buf, key...)
  	return buf
  }
docs: // secureKey returns the database key for the preimage of key, as an ephemeral
  // buffer. The caller must not hold onto the return value because it will become
  // invalid on the next call.