// iterate safely (& quickly) in the face of concurrent modifications
static <A, B> IterableIterator<B> concurrentlyIterateValues(NavigableMap<A, B> map) {
  ret concurrentlyIterateValues(map, map);
}

static <A, B> IterableIterator<B> concurrentlyIterateValues(final NavigableMap<A, B> map, O mutex) {
  ret iteratorFromFunction(new F0<B>() {
    Iterator<A> it = keys(map).iterator();
    A key;
    
    B get() {
      synchronized(mutex) {
        try {
          if (!it.hasNext()) null;
          ret map.get(key = it.next());
        } catch (ConcurrentModificationException e) {
          print("Re-iterating");
          it = map.tailMap(key, false).keySet().iterator();
          if (!it.hasNext()) null;
          ret map.get(key = it.next()); // Can't throw another exception
        }
      }
    }
  });
}