// predicates: L<F1<S, Bool>>
static L<Int> indicesOfTokenPredicateSequence(LS tok, L predicates) {
  int n = l(predicates), nn = l(tok)-(n-1)*2;
  new L<Int> out;
  search: for (int i = 1; i < nn; i += 2) {
    for j to n:
      if (!isTrue(callF(predicates.get(j), tok.get(i+j*2)))) continue search;
    out.add(i);
  }
  ret out;
}