code: func extractIdentityID(ctx context.Context) (string, error) {
  	token := goajwt.ContextJWT(ctx)
  	if token == nil {
  		return "", errors.New("Missing token")
  	}
  	id := token.Claims.(jwt.MapClaims)["sub"]
  	if id == nil {
  		return "", errors.New("Missing sub")
  	}
  
  	return id.(string), nil
  }
docs: // extractIdentityID obtains the identity ID out of the authentication context