// use "*" as wildcard
// may return null, and returned list is "volatile" (only valid as long as the index is not changed)
static LLS positionalTokenIndex_filter(PositionalTokenIndex idx, LS tok) {
  int cutOff = 1; // if we find a list shorter than this, we just return it. must be at least 1
  LLS shortestList = null;
  for (int i = 1; i < l(tok); i += 2) {
    S t = tok.get(i);
    if (!eq(t, "*")) {
      LLS list = idx.byToken(i/2, t);
      if (l(list) < cutOff) ret list;
      if (shortestList == null || l(list) < l(shortestList))
        shortestList = list;
    }
  }
  if (shortestList == null) // pattern must be all wildcards - return full list
    ret idx.fullList();
  ret shortestList;
}