docs: def get_state_machine_selection(self):
          """An abstract getter method for state machine selection
  
          The method maybe has to be re-implemented by inherit classes and hands generally a filtered set of
          selected elements.
  
          :return: selection object it self, filtered set of selected elements
          :rtype: rafcon.gui.selection.Selection, set
          """
          sm_selection = self.model.get_state_machine_m().selection if self.model.get_state_machine_m() else None
          return sm_selection, sm_selection.get_selected_elements_of_core_class(self.CORE_ELEMENT_CLASS) if sm_selection else set()