// 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, Int> getBracketMap(L tok, Collection<S> opening, Collection<S> closing) {
  new TreeMap<Int, Int> map;
  new L<int> stack;
  for (int i = 1; i < l(tok); i+= 2) {
    if (opening.contains(tok.get(i)))
      stack.add(i);
    else if (closing.contains(tok.get(i))) {
      if (!empty(stack))
        map.put(liftLast(stack), i);
    }
  }
  ret map;
}

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