static ItIt<LS> ai_spreadCertainIndices_2_iterator(LS tokPat, LS tokS, L<Int> indices) {

  int extra = (l(tokS)-l(tokPat))/2;

  if (extra < 0 || empty(indices)) ret emptyItIt();

  ret mapI(allSpreads(extra, l(indices)), spread -> {

    new LS tok;

    int pos = 0, iSpread = 0, iIndices = 0;

    for i over tokPat:

      if (!eq(get(indices, iIndices), i))

        tok.add(tokS.get(pos++));

      else {

        ++iIndices;

        int take = spread.get(iSpread++)*2;

        tok.add(joinSubList(tokS, pos, pos+take+1));

        pos += take+1;

      }

    ret tok;

  });

}