code: func Nkey(pubKey string, sigCB SignatureHandler) Option {
  	return func(o *Options) error {
  		o.Nkey = pubKey
  		o.SignatureCB = sigCB
  		if pubKey != "" && sigCB == nil {
  			return ErrNkeyButNoSigCB
  		}
  		return nil
  	}
  }
docs: // Nkey will set the public Nkey and the signature callback to
  // sign the server nonce.