// i must point at the (possibly imaginary) opening bracket ("{")
// index returned is index of closing bracket + 1 (or l(tok))
static int tok_findEndOfBlock_IContentsIndexedList2(IContentsIndexedList2<S> tok, int i) {
  int n = l(tok);
  
  // find opening & closing brackets after start index
  
  TreeSet<HasIndex> openers = tok.indicesOf_treeSetOfHasIndex("{");
  TreeSet<HasIndex> closers = tok.indicesOf_treeSetOfHasIndex("}");

  // now step through both sets
  
  int level = 1;
  int nextOpener = toIntOr(openers.higher(i), n);
  int nextCloser = toIntOr(closers.higher(i), n);
  while true {
    if (nextOpener < nextCloser) {
      i = nextOpener;
      ++level;
      nextOpener = toIntOr(openers.higher(i), n);
    } else if (nextOpener > nextCloser) {
      i = nextCloser;
      if (--level == 0)
        ret i+1;
      nextCloser = toIntOr(closers.higher(i), n);
    } else // opener and closer in one place!? we must be at the end.
      ret i;
  }
}