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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _unregister_lookup(cls, lookup, lookup_name=None):
 """ Remove given lookup from cls lookups. For use in tests only as it's not thread-safe. """

        if lookup_name is None:
            lookup_name = lookup.lookup_name
        del cls.class_lookups[lookup_name]