This function returns the unsat core from the backend solver.

        :param extra_constraints:   Extra constraints (as ASTs) to add to the solver for this solve.
        :return: The unsat core.

def unsat_core(self, extra_constraints=()):
        """
        This function returns the unsat core from the backend solver.

        :param extra_constraints:   Extra constraints (as ASTs) to add to the solver for this solve.
        :return: The unsat core.
        """
        if o.CONSTRAINT_TRACKING_IN_SOLVER not in self.state.options:
            raise SimSolverOptionError('CONSTRAINT_TRACKING_IN_SOLVER must be enabled before calling unsat_core().')
        return self._solver.unsat_core(extra_constraints=extra_constraints)