public SignInViewModel(
			StageManager navigator, 
			SessionState sessionState,
			AuthenticationService authenticator,
			SessionService sessionService,
			CourseSettingsService courseSettingsService)
		{
			this.navigator = navigator;
			this.sessionState = sessionState;
			this.authenticator = authenticator;
			this.sessionService = sessionService;
			this.courseSettingsService = courseSettingsService;
		}
		public MainMenuViewModel(
			StageManager navigator, 
			SessionState sessionState,
			AuthenticationService authenticator, 
			ActivityService activityService,
			ActivityLauncher activityLauncher)
		{
			this.navigator = navigator;
			this.sessionState = sessionState;
			this.authenticator = authenticator;
			this.activityService = activityService;
			this.activityLauncher = activityLauncher;
		}
		public WebActivityViewModel(SessionState sessionState, StageManager navigator, ActivityService activityService) : base(navigator, activityService)
		{
			Contract.ArgumentNotNull("sessionState", sessionState);

			this.sessionState = sessionState;
		}