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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 psd(self):
 """ A pyCBC FrequencySeries holding the appropriate PSD. Return the PSD used in the metric calculation. """

        if not self._psd:
            errMsg = "The PSD has not been set in the metricParameters "
            errMsg += "instance."
            raise ValueError(errMsg)
        return self._psd