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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 base_flow_index(q, freq='YS'):
 r"""Base flow index Return the base flow index, defined as the minimum 7-day average flow divided by the mean flow. Parameters q : xarray.DataArray Rate of river discharge [m³/s] freq : str, optional Resampling frequency Returns ------- xarray.DataArrray Base flow index. Notes ----- Let :math:`\mathbf{q}=q_0, q_1, \ldots, q_n` be the sequence of daily discharge and :math:`\overline{\mathbf{q}}` the mean flow over the period. The base flow index is given by: .. math:: \frac{\min(\mathrm{CMA}_7(\mathbf{q}))}{\overline{\mathbf{q}}} where :math:`\mathrm{CMA}_7` is the seven days moving average of the daily flow: .. math:: \mathrm{CMA}_7(q_i) = \frac{\sum_{j=i-3}^{i+3} q_j}{7} """


    m7 = q.rolling(time=7, center=True).mean().resample(time=freq)
    mq = q.resample(time=freq)

    m7m = m7.min(dim='time')
    return m7m / mq.mean(dim='time')