code: func HDPrivateKeyToPublicKeyID(id []byte) ([]byte, error) {
  	if len(id) != 4 {
  		return nil, ErrUnknownHDKeyID
  	}
  
  	var key [4]byte
  	copy(key[:], id)
  	pubBytes, ok := hdPrivToPubKeyIDs[key]
  	if !ok {
  		return nil, ErrUnknownHDKeyID
  	}
  
  	return pubBytes, nil
  }
docs: // HDPrivateKeyToPublicKeyID accepts a private hierarchical deterministic
  // extended key id and returns the associated public key id.  When the provided
  // id is not registered, the ErrUnknownHDKeyID error will be returned.