// map: index of opening bracket -> index of closing bracket
static Map<Int> curlyBracketMap(L<S> tok) {
  new TreeMap<Int, Int> map;
  new L<int> stack;
  int n = l(tok);
  for (int i = 1; i < n; i += 2) {
    S t = tok.get(i);
    if (eq(t, "{"))
      stack.add(i);
    else if (eq(t, "}") && nempty(stack))
      map.put(liftLast(stack), i);
  }
  ret map;
}