def gen_calendars(start, stop, critical_dates):
    """
    Generate calendars to use as inputs.
    """
    all_dates = pd.date_range(start, stop, tz='utc')
    for to_drop in map(list, powerset(critical_dates)):
        # Have to yield tuples.
        yield (all_dates.drop(to_drop),)

    # Also test with the trading calendar.
    trading_days = get_calendar("NYSE").all_days
    yield (trading_days[trading_days.slice_indexer(start, stop)],)