code: def selected_expander(self, selected_expander):
      if selected_expander in self.expander_pool:
          self._selected_expander = selected_expander
      else:
          if len(self.expander_pool) > 0:
              default_expander = next(iter(self.expander_pool))
              logger.info(
                  ">> Expander you are attempting to use %s does not exist in expander pool. Using the following default expander instead %s ",
                  selected_expander,
                  default_expander,
              )
              self._selected_expander = default_expander
          else:
              logger.info(
                  ">> No expander has been specified in config.yaml."
              )
              self._selected_expander = None
  
imports: from neuralqa.expander import MLMExpander
  import logging
label_desc: Use image node_id 4 for calling the ExpanderPool obj's underlying member method code with example usage: obj.selected_expander(selected_expander) without return types