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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 frost_days(tasmin, freq='YS'):
 r"""Frost days index Number of days where daily minimum temperatures are below 0℃. Parameters tasmin : xarray.DataArray Minimum daily temperature [℃] or [K] freq : str, optional Resampling frequency Returns ------- xarray.DataArray Frost days index. Notes ----- Let :math:`TN_{ij}` be the daily minimum temperature at day :math:`i` of period :math:`j`. Then counted is the number of days where: .. math:: TN_{ij} < 0℃ """

    tu = units.parse_units(tasmin.attrs['units'].replace('-', '**-'))
    fu = 'degC'
    frz = 0
    if fu != tu:
        frz = units.convert(frz, fu, tu)
    f = (tasmin < frz) * 1
    return f.resample(time=freq).sum(dim='time')