How do you clear history?

[xCONFLiCTiONx]

I see no way to clear history of the toast notifications. If they user does not activate the toast then it just lingers in the notification area. In most cases that would be best but in the case of my application which logs people out after a certain amount of time and a toast alerts them this is happening then they log back in and that notification is still there.

ToastNotificationManager.History.Clear(); is what I have read up on but not working. Sorry, this is above my head.