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

// TODO: could use iterator to be even faster

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;
  HasIndex start = new(i);
  HasIndex nextOpener = openers.higher(start);
  HasIndex nextCloser = closers.higher(start);

  while true {
    if (nextCloser == null) ret n; // block isn't closed
    
    if (nextOpener != null && nextOpener.idx < nextCloser.idx) {
      // process opener
      ++level;
      nextOpener = openers.higher(nextOpener);
    } else {
      // process closer
      if (--level == 0)
        ret nextCloser.idx+1;
      nextCloser = closers.higher(nextCloser);
    }
  }
}

Began life as a copy of #1025221