Second task to be performed by a pool of subprocesses before
        the cluster labels and cluster center indices can be identified.

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