code: func UserCredentials(userOrChainedFile string, seedFiles ...string) Option {
  	userCB := func() (string, error) {
  		return userFromFile(userOrChainedFile)
  	}
  	var keyFile string
  	if len(seedFiles) > 0 {
  		keyFile = seedFiles[0]
  	} else {
  		keyFile = userOrChainedFile
  	}
  	sigCB := func(nonce []byte) ([]byte, error) {
  		return sigHandler(nonce, keyFile)
  	}
  	return UserJWT(userCB, sigCB)
  }
docs: // UserCredentials is a convenience function that takes a filename
  // for a user's JWT and a filename for the user's private Nkey seed.