// collects into l
static <A> Collector<A> limitedListCollector(final Collection<A> l, final int max) {
  ret new Collector<A>() {
    bool full() {
      ret l(l) >= max;
    }
    
    bool add(A a) {
      if (full()) false;
      l.add(a);
      ret full();
    }
    
    bool contains(A a) {
      ret l.contains(a);
    }
  };
}