docs: def _find_links(k_vec, sfx, rsfx, k):
      """Find sfx/rsfx recursively."""
      k_vec.sort()
      if 0 in k_vec:
          return k_vec
      else:
          if sfx[k] not in k_vec:
              k_vec.append(sfx[k])
          for i in range(len(rsfx[k])):
              if rsfx[k][i] not in k_vec:
                  k_vec.append(rsfx[k][i])
          for i in range(len(k_vec)):
              k_vec = _find_links(k_vec, sfx, rsfx, k_vec[i])
              if 0 in k_vec:
                  break
          return k_vec