code: def release_references(self, keys):
          
          keys = set(self.referenced_by) & set(keys)
          for key in keys:
              self.referenced_by.pop(key)
docs: Non-recursively indicate that an iterable of _ReferenceKey no longer
          exist. Unknown keys are ignored.
  
          :param Iterable[_ReferenceKey] keys: The keys to drop.