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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 coalesce(self):
 """Coalesce the segments for this flag. This method does two things: - `coalesces <SegmentList.coalesce>` the `~DataQualityFlag.known` and `~DataQualityFlag.active` segment lists - forces the `active` segments to be a proper subset of the `known` segments .. note:: this operations is performed in-place. Returns ------- self a view of this flag, not a copy. """

        self.known = self.known.coalesce()
        self.active = self.active.coalesce()
        self.active = (self.known & self.active).coalesce()
        return self