The slice of the stack following the topmost markobject is taken as
      an alternating sequence of keys and values, added to the dict
      immediately under the topmost markobject.  Everything at and after the
      topmost markobject is popped, leaving the mutated dict at the top
      of the stack.

      Stack before:  ... pydict markobject key_1 value_1 ... key_n value_n
      Stack after:   ... pydict

      where pydict has been modified via pydict[key_i] = value_i for i in
      1, 2, ..., n, and in that order.
      Ú	EMPTY_SETô