code: def __get_ra_index_indices(self):
          
          fragment_indices = []
          for idx, cumlen in enumerate(self._cumulative_lengths):
              cumlen_prev = self._cumulative_lengths[idx - 1] if idx > 0 else 0
              fragment_indices.append([np.argwhere(
                  np.logical_and(self.ra_indices >= cumlen_prev, self.ra_indices < cumlen)
              )])
          return fragment_indices
docs: Returns a list containing indices of the ra_index array, which correspond to the separate trajectory fragments,
          i.e., ra_indices[fragment_indices[itraj]] are the ra indices for itraj (plus some offset by
          cumulative length)