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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 contributions(self):
 """Apply a datetime filter against the contributor's contribution queryset."""

        if self._contributions is None:
            self._contributions = self.contributor.contributions.filter(
                content__published__gte=self.start,
                content__published__lt=self.end
            )
        return self._contributions