code: func getUserAddr(digest common.Hash, signature Signature) (common.Address, error) {
  	pub, err := crypto.SigToPub(digest.Bytes(), signature[:])
  	if err != nil {
  		return common.Address{}, err
  	}
  	return crypto.PubkeyToAddress(*pub), nil
  }
docs: // getUserAddr extracts the address of the feed update signer