// i must point at the (possibly imaginary) opening bracket ("{")
// index returned is index of closing bracket + 1 (or l(tok))

// version using iterator (much faster than v1!)

static int tok_findEndOfBlock_IContentsIndexedList2_v2(IContentsIndexedList2<S> tok, int i) {
  int n = l(tok);
  
  // find opening & closing brackets after start index
  
  HasIndex start = new(i);
  TreeSet<HasIndex> openers = tok.indicesOf_treeSetOfHasIndex("{");
  Iterator<HasIndex> iOpener = openers?.tailSet(start, false).iterator();
  TreeSet<HasIndex> closers = tok.indicesOf_treeSetOfHasIndex("}");
  Iterator<HasIndex> iCloser = closers?.tailSet(start, false).iterator();

  // now step through both sets
  
  int level = 1;
  HasIndex nextOpener = iOpener != null && iOpener.hasNext() ? iOpener.next() : null;
  HasIndex nextCloser = iCloser != null && iCloser.hasNext() ? iCloser.next() : null;

  while true {
    if (nextCloser == null) ret n; // block isn't closed
    
    if (nextOpener != null && nextOpener.idx < nextCloser.idx) {
      // process opener
      ++level;
      nextOpener = iOpener.hasNext() ? iOpener.next() : null;
    } else {
      // process closer
      if (--level == 0)
        ret nextCloser.idx+1;
      nextCloser = iCloser.hasNext() ? iCloser.next() : null;
    }
  }
}