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;
  });
}

Began life as a copy of #1021071