code: public static Actor user(String zoneId, String userId) {
  		Assert.notNull(zoneId, "zoneId must not be null");
  		Assert.notNull(userId, "userId must not be null");
  		return new Actor(USER, zoneId + "/" + userId);
  	}
docs: Create a user identifier. A user is identified by a GUID generated by UAA when
  a user account is created and the ID of the identity zone the user was created in.
  
  @param zoneId the UAA identity zone ID
  @param userId the UAA user GUID
  @return the created {@literal Actor}