def revert(self, a):
        if self.is_one(a):
            return a
        else:
            raise NotReversible('only unity is reversible in a ring')
Test whether hashed columns are mapped to correct HC dimensions.
dataset/ETHPy150Open sympy/sympy/sympy/polys/domains/ring.py/Ring.revert--crossover with--dataset/ETHPy150Open uwescience/raco/raco/nary_join_rules_test.py/testNaryJoin.test_hashed_column_mapping