<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def tx_days_above(tasmax, thresh='25.0 degC', freq='YS'):
 r"""Number of summer days Number of days where daily maximum temperature exceed a threshold. Parameters tasmax : xarray.DataArray Maximum daily temperature [℃] or [K] thresh : str Threshold temperature on which to base evaluation [℃] or [K]. Default: '25 degC'. freq : str, optional Resampling frequency Returns ------- xarray.DataArray Number of summer days. Notes ----- Let :math:`TX_{ij}` be the daily maximum temperature at day :math:`i` of period :math:`j`. Then counted is the number of days where: .. math:: TX_{ij} > Threshold [℃] """

    thresh = utils.convert_units_to(thresh, tasmax)
    f = (tasmax > (thresh)) * 1
    return f.resample(time=freq).sum(dim='time')