code: def _remove_refs(self, keys):
          
          # remove direct refs
          for key in keys:
              del self.relations[key]
          # then remove all entries from each child
          for cached in self.relations.values():
              cached.release_references(keys)
docs: Removes all references to all entries in keys. This does not
          cascade!
  
          :param Iterable[_ReferenceKey] keys: The keys to remove.