class_docstr: Single global state to keep track of scope stacks.
code: def current_scope(self):
      if len(self._stack):
          return self._stack[-1]
      return self.global_scope
  
imports: from collections import defaultdict
  from collections.abc import Iterable
  from functools import singledispatch
  import pyro
  from pyro.poutine.handlers import _make_handler
  from pyro.poutine.reentrant_messenger import ReentrantMessenger
  from pyro.poutine.runtime import effectful
label_desc: Use image node_id 4 for calling the ScopeStack obj's underlying member method code with example usage: obj.current_scope() and returns: self, self