// map: index of opening bracket -> index of closing bracket
static Map<Int, Int> getBracketMap(L tok) {
  ret getBracketMap(tok, getBracketMap_opening, getBracketMap_closing);
}

static Map<Int> getBracketMap(L tok, Collection<S> opening, Collection<S> closing) {
  ret getBracketMap(tok, opening, closing, 0, l(tok));
}

static Map<Int> getBracketMap(L tok, Cl<S> opening, Cl<S> closing, int from, int to) {
  new TreeMap<Int> map;
  new L<Int> stack;
  for (int i = from|1; i < to; i+= 2) {
    O t = tok.get(i);
    if (opening.contains(t))
      stack.add(i);
    else if (closing.contains(t))
      if (!empty(stack))
        map.put(liftLast(stack), i);
  }
  ret map;
}

static Map<Int> getBracketMap(LS tok, IPred<S> opening, IPred<S> closing) {
  ret getBracketMap(tok, opening, closing, 0, l(tok));
}

static Map<Int> getBracketMap(LS tok, IPred<S> opening, IPred<S> closing, int from, int to) {
  new TreeMap<Int> map;
  new L<Int> stack;
  for (int i = from|1; i < to; i+= 2) {
    S t = tok.get(i);
    if (opening.get(t))
      stack.add(i);
    else if (closing.get(t))
      if (!empty(stack))
        map.put(liftLast(stack), i);
  }
  ret map;
}

static Set<S> getBracketMap_opening = lithashset("{", "(");
static Set<S> getBracketMap_closing = lithashset("}", ")");