docs: def cluster_labels_B(hdf5_file, s_reduced, lock, I, ii, iix, rows_slice):
      """Second task to be performed by a pool of subprocesses before
          the cluster labels and cluster center indices can be identified.
      """
  
      with Worker.hdf5_lock:
          with tables.open_file(hdf5_file, 'r+') as fileh:
              S = fileh.root.aff_prop_group.similarities
              s = S[rows_slice, ...]
             
      s = s[:, ii]
      s = s[iix[rows_slice]]
      
      with lock:                
          s_reduced += s[:].sum(axis = 0)
          
      del s