def _undo(self):
        # Undo the variable settings
        for lit in self._current_level.var_settings:
            self.var_settings.remove(lit)
            self.heur_lit_unset(lit)
            self.variable_set[abs(lit)] = False

        # Pop the level off the stack
        self.levels.pop()

    #########################
    #      Propagation      #
    #########################
_undo the changes of the most recent decision level.
dataset/ETHPy150Open sympy/sympy/sympy/logic/algorithms/dpll2.py/SATSolver._undo