docs: 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