An object that is notified as elements are pushed and popped and typically controls a "named condition" (which is a count associated with a name).
Returns a "fresh" copy of this item. May be this if immutable.
return true if no more notifications are desired
Depth is 0 for element itself and its siblings, 1 for direct children and -1 for the parent.
An object that is notified as elements are pushed and popped and typically controls a "named condition" (which is a count associated with a name).