code: def add(self, name, cfg, expand=False):
          
          self._d[name] = cfg
          if expand:
              self.expand(name)
          return self
docs: Add a configuration object.
  
          :param name: Name for later retrieval
          :param cfg: Configuration object
          :param expand: Flag for adding sub-configs for each sub-collection.
                         See discussion in method doc.
          :return: self, for chaining
          :raises: CreateQueryEngineError (only if expand=True)