Retrieve a (any) object from the set. This removes an element from the set and returns it.
void add All object;
Add the object to the receiving set.
void remove All object;
Remove elt from the receiving set, if present.
void gc_mark_containers pre [self isContainer];
This method is invoked by the garbage collector to have the set containing all containers make those containers mark their elements. This method relies on the container containing the containers be itself a container.