code: func (c *ServerContext) Update(authenticate []byte) error {
  	authCompleted, n, err := updateContext(c.sctxt, nil, authenticate)
  	if err != nil {
  		return err
  	}
  	if !authCompleted {
  		return errors.New("ntlm authentication should be completed now")
  	}
  	if n != 0 {
  		return errors.New("ntlm token should be empty now")
  	}
  	return nil
  }
docs: // Update completes server part of NTLM negotiation c. It uses
  // authenticate message received from the client.