def get_reverse_index(self, base_index):
        """Get index into this segment's data given the index into the base data

        Raises IndexError if the base index doesn't map to anything in this
        segment's data
        """
        r = self.reverse_index_mapping[base_index]
        if r < 0:
            raise IndexError("index %d not mapped in this segment" % base_index)
        return r
Get index into this segment's data given the index into the base data

        Raises IndexError if the base index doesn't map to anything in this
        segment's data