def _check_info(info, sss=True, tsss=True, calibration=True, ctc=True):
    """Ensure that Maxwell filtering has not been applied yet."""
    for ent in info['proc_history']:
        for msg, key, doing in (('SSS', 'sss_info', sss), ('tSSS', 'max_st',
            tsss), ('fine calibration', 'sss_cal', calibration), (
            'cross-talk cancellation', 'sss_ctc', ctc)):
            if not doing:
                continue
            if len(ent['max_info'][key]) > 0:
                raise RuntimeError(
                    'Maxwell filtering %s step has already been applied, cannot reapply'
                     % msg)
