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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 ensure_treasury_data(symbol, first_date, last_date, now, environ=None):
 """ Ensure we have treasury data from treasury module associated with `symbol`. Parameters symbol : str Benchmark symbol for which we're loading associated treasury curves. first_date : pd.Timestamp First date required to be in the cache. last_date : pd.Timestamp Last date required to be in the cache. now : pd.Timestamp The current time. This is used to prevent repeated attempts to re-download data that isn't available due to scheduling quirks or other failures. We attempt to download data unless we already have data stored in the cache for `module_name` whose first entry is before or on `first_date` and whose last entry is on or after `last_date`. If we perform a download and the cache criteria are not satisfied, we wait at least one hour before attempting a redownload. This is determined by comparing the current time to the result of os.path.getmtime on the cache path. """

    loader_module, filename, source = INDEX_MAPPING.get(
        symbol, INDEX_MAPPING['SPY'],
    )
    first_date = max(first_date, loader_module.earliest_possible_date())

    data = _load_cached_data(filename, first_date, last_date, now, 'treasury',
                             environ)
    if data is not None:
        return data

    # If no cached data was found or it was missing any dates then download the
    # necessary data.
    logger.info(
        ('Downloading treasury data for {symbol!r} '
            'from {first_date} to {last_date}'),
        symbol=symbol,
        first_date=first_date,
        last_date=last_date
    )

    try:
        data = loader_module.get_treasury_data(first_date, last_date)
        data.to_csv(get_data_filepath(filename, environ))
    except (OSError, IOError, HTTPError):
        logger.exception('failed to cache treasury data')
    if not has_data_for_dates(data, first_date, last_date):
        logger.warn(
            ("Still don't have expected treasury data for {symbol!r} "
                "from {first_date} to {last_date} after redownload!"),
            symbol=symbol,
            first_date=first_date,
            last_date=last_date
        )
    return data