docs: def add_preference(hdf5_file, preference):
      """Assign the value 'preference' to the diagonal entries
          of the matrix of similarities stored in the HDF5 data structure 
          at 'hdf5_file'.
      """
  
      Worker.hdf5_lock.acquire()
      
      with tables.open_file(hdf5_file, 'r+') as fileh:
          S = fileh.root.aff_prop_group.similarities
          diag_ind = np.diag_indices(S.nrows)
          S[diag_ind] = preference
          
      Worker.hdf5_lock.release()