code: def configure_retrieve(self, ns, definition):
          
          request_schema = definition.request_schema or Schema()
  
          @self.add_route(ns.instance_path, Operation.Retrieve, ns)
          @qs(request_schema)
          @response(definition.response_schema)
          @wraps(definition.func)
          def retrieve(**path_data):
              headers = dict()
              request_data = load_query_string_data(request_schema)
              response_data = require_response_data(definition.func(**merge_data(path_data, request_data)))
              definition.header_func(headers, response_data)
              response_format = self.negotiate_response_content(definition.response_formats)
              return dump_response_data(
                  definition.response_schema,
                  response_data,
                  headers=headers,
                  response_format=response_format,
              )
  
          retrieve.__doc__ = "Retrieve a {} by id".format(ns.subject_name)
docs: Register a retrieve endpoint.
  
          The definition's func should be a retrieve function, which must:
          - accept kwargs for path data
          - return an item or falsey
  
          :param ns: the namespace
          :param definition: the endpoint definition