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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def validate( feed: "Feed", *, as_df: bool = True, include_warnings: bool = True ) -> Union[List, DataFrame]: """ Check whether the given feed satisfies the GTFS. Parameters feed : Feed as_df : boolean If ``True``, then return the resulting report as a DataFrame; otherwise return the result as a list include_warnings : boolean If ``True``, then include problems of types ``'error'`` and ``'warning'``; otherwise, only return problems of type ``'error'`` Returns ------- list or DataFrame Run all the table-checking functions: :func:`check_agency`, :func:`check_calendar`, etc. This yields a possibly empty list of items [problem type, message, table, rows]. If ``as_df``, then format the error list as a DataFrame with the columns - ``'type'``: 'error' or 'warning'; 'error' means the GTFS is violated; 'warning' means there is a problem but it's not a GTFS violation - ``'message'``: description of the problem - ``'table'``: table in which problem occurs, e.g. 'routes' - ``'rows'``: rows of the table's DataFrame where problem occurs Return early if the feed is missing required tables or required columns. Notes ----- - This function interprets the GTFS liberally, classifying problems as warnings rather than errors where the GTFS is unclear. For example if a trip_id listed in the trips table is not listed in the stop times table (a trip with no stop times), then that's a warning and not an error. - Timing benchmark: on a 2.80 GHz processor machine with 16 GB of memory, this function checks `this 31 MB Southeast Queensland feed <http://transitfeeds.com/p/translink/21/20170310>`_ in 22 seconds, including warnings. """

    problems = []

    # Check for invalid columns and check the required tables
    checkers = [
        "check_agency",
        "check_calendar",
        "check_calendar_dates",
        "check_fare_attributes",
        "check_fare_rules",
        "check_feed_info",
        "check_frequencies",
        "check_routes",
        "check_shapes",
        "check_stops",
        "check_stop_times",
        "check_transfers",
        "check_trips",
    ]
    for checker in checkers:
        problems.extend(
            globals()[checker](feed, include_warnings=include_warnings)
        )

    # Check calendar/calendar_dates combo
    if feed.calendar is None and feed.calendar_dates is None:
        problems.append(
            ["error", "Missing both tables", "calendar & calendar_dates", []]
        )

    return format_problems(problems, as_df=as_df)