def _check_opt_conf(parser):
    """
    Check for particular configuration flags.
    
    Parameters
    ----------
    parser : argparse.ArgumentParser
        A parser with a 'workflow_config' item inside.
    
    Returns
    -------
    parser : argparse.ArgumentParser
        If parser.workflow_config is None, returns the unmodified
        input parameter. Otherwise, set its items based on the flag.
    
    Raises
    ------
    NotImplementedError
        If parser.workflow_config is not equal to a supported string.
        Which shouldn't happen, because this function should not be 
        called on its own.
    """
    if parser.workflow_config is not None:
        if parser.workflow_config == 'brightspin':
            parser.lag_max = 9
            parser.lag_step = 0.3
            parser.lagged_regression = True
            parser.run_conv = True
            parser.apply_filter = False
        elif parser.workflow_config == 'brightspin-clinical':
            parser.lag_max = 20
            parser.lag_step = 0.3
            parser.lagged_regression = True
            parser.run_conv = True
            parser.apply_filter = False
        elif parser.workflow_config == 'baltimore':
            parser.run_conv = False
            parser.apply_filter = True
            parser.lowcut = 0.02
            parser.highcut = 0.04
            parser.fname_co2 = ''
            parser.lagged_regression = False
        elif parser.workflow_config == 'baltimore-lag':
            parser.run_conv = False
            parser.apply_filter = True
            parser.lowcut = 0.02
            parser.highcut = 0.04
            parser.fname_co2 = ''
            parser.lagged_regression = True
        else:
            raise NotImplementedError(f'{parser.workflow_config} is not configured. '
                                      'In fact, you shouldn\'t see this message at all.')

    del parser.workflow_config
    return parser