Return the default frequency.

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