code: def get_elements(self, filter_cls, elem_id=None):
          
          result = []
          if elem_id is not None:
              try:
                  result = [self._class_collection_map[filter_cls][elem_id]]
              except KeyError:
                  result = []
          else:
              for e in self._class_collection_map[filter_cls].values():
                  result.append(e)
          return result
docs: Get a list of elements from the result and filter the element type by a class.
  
          :param filter_cls:
          :param elem_id: ID of the object
          :type elem_id: Integer
          :return: List of available elements
          :rtype: List