Each new topic added to the tree is assigned a default time-to-live (t2l) value (a long integer) measured in days. Each day that the nominated topic does not receive some form of community support (described below) the t2l value is decremented. If it reaches zero, then the nominated item is removed from the set (perhaps after archiving for audit trail support). If, on the other hand, the community provides support for the item, then it's t2l value is incremented by a non-unit value and it continues to live in the structure. In fact, when the t2l value exceeds some agreed upon limit, the item can be made permanent. This means that the community has agreed that the item is a genuine and important sub-topic and that it is at the appropriate level in the tree.