def _assert_n_free(raw_sss, lower, upper=None):
    """Helper to check the DOF"""
    upper = lower if upper is None else upper
    n_free = raw_sss.info['proc_history'][0]['max_info']['sss_info']['nfree']
    assert_true(lower <= n_free <= upper,
                (lower, n_free, upper) % 'nfree fail: %s <= %s <= %s')
dataset/ETHPy150Open mne-tools/mne-python/mne/preprocessing/tests/test_maxwell.py/_assert_n_free/swapped operands of `%`