This function returns the unsat core from the backend solver.

        :param s: A backend solver object.
        :return: The unsat core.

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

        :param s: A backend solver object.
        :return: The unsat core.
        """

        return [ self._abstract(core) for core in self._unsat_core(s) ]