he set immediately under the topmost
      markobject.  Everything at and after the topmost markobject is popped,
      leaving the mutated set at the top of the stack.

      Stack before:  ... pyset markobject item_1 ... item_n
      Stack after:   ... pyset

      where pyset has been modified via pyset.add(item_i) = item_i for i in
      1, 2, ..., n, and in that order.
      Z	FROZENSETő