docs: def extend_selection():
      """Checks is the selection is to be extended
  
      The selection is to be extended, if a special modifier key (typically <Ctrl>) is being pressed.
  
      :return: If to extend the selection
      :rtype: True
      """
      from rafcon.gui.singleton import main_window_controller
      currently_pressed_keys = main_window_controller.currently_pressed_keys if main_window_controller else set()
      if any(key in currently_pressed_keys for key in [constants.EXTEND_SELECTION_KEY,
                                                       constants.EXTEND_SELECTION_KEY_ALT]):
          return True
      return False