code: func (db *Database) Selector(id []byte) (string, error) {
  	if len(id) < 4 {
  		return "", fmt.Errorf("expected 4-byte id, got %d", len(id))
  	}
  	sig := hex.EncodeToString(id[:4])
  	if selector, exists := db.embedded[sig]; exists {
  		return selector, nil
  	}
  	if selector, exists := db.custom[sig]; exists {
  		return selector, nil
  	}
  	return "", fmt.Errorf("signature %v not found", sig)
  }
docs: // Selector checks the given 4byte ID against the known ABI methods.
  //
  // This method does not validate the match, it's assumed the caller will do.