// f: func(L<S>) -> L<S>
static L<S> reTok_modify(L<S> tok, int i, int j, O f) {
  // extend i to an "N" token
  // and j to "C" (so j-1 is an "N" token)
  i = i & ~1;
  j = j | 1;
  
  L<S> t = javaTok(join(subList(tok, i, j)));#
  if (f != null)
    t = (L<S>) callF(f, t);
  replaceListPart(tok, i, j, t);
  
  ret tok;
}