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