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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 get_treasury_yield(start=None, end=None, period='3MO'):
 """ Load treasury yields from FRED. Parameters start : date, optional Earliest date to fetch data for. Defaults to earliest date available. end : date, optional Latest date to fetch data for. Defaults to latest date available. period : {'1MO', '3MO', '6MO', 1', '5', '10'}, optional Which maturity to use. Returns ------- pd.Series Annual treasury yield for every day. """


    if start is None:
        start = '1/1/1970'
    if end is None:
        end = _1_bday_ago()

    treasury = web.DataReader("DGS3{}".format(period), "fred",
                              start, end)

    treasury = treasury.ffill()

    return treasury