ost markobject is taken as
      a sequence of items, added to the 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