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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 peak_generation(self):
 """ Cumulative peak generation capacity of generators of this grid Returns ------- float Ad-hoc calculated or cached peak generation capacity """

        if self._peak_generation is None:
            self._peak_generation = sum(
                [gen.nominal_capacity
                 for gen in self.generators])

        return self._peak_generation