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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 update(self) -> None: """Update subclass of |RelSubweightsMixin| based on `refweights`."""

        mask = self.mask
        weights = self.refweights[mask]
        self[~mask] = numpy.nan
        self[mask] = weights/numpy.sum(weights)