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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 default_freq(**indexer):
 """Return the default frequency."""

    freq = 'AS-JAN'
    if indexer:
        if 'DJF' in indexer.values():
            freq = 'AS-DEC'
        if 'month' in indexer and sorted(indexer.values()) != indexer.values():
            raise (NotImplementedError)

    return freq