code: def undo_nested_group(self):
          
          if self._undoing or self._redoing:
              raise RuntimeError
          if self._open:
              group = self._open.pop()
          elif self._undo:
              group = self._undo.pop()
          else:
              return
  
          self._undoing = True
  
          self.begin_grouping()
          group.perform()
          self.set_action_name(group.name)
          self.end_grouping()
  
          self._undoing = False
  
          self.notify()
docs: Performs the last group opened, or the top group on the undo stack.
          Creates a redo group with the same name.