def __delattr__(self, key):
        with self.lock:
            del self.tbl[key]
given a with_polymorphic() argument, return the set of mappers it
dataset/ETHPy150Open ejeschke/ginga/ginga/misc/Bunch.py/threadSafeBunch.__delattr__--crossover with--dataset/ETHPy150Open goFrendiAsgard/kokoropy/kokoropy/packages/sqlalchemy/orm/mapper.py/Mapper._mappers_from_spec