// iterate safely (& quickly) in the face of concurrent modifications
static <A, B> IterableIterator<A> concurrentlyIterateKeys(final NavigableMap<A, B> map) {
  ret iteratorFromFunction(new F0<A>() {
    Iterator<A> it = keys(map).iterator();
    A key;
    
    A get() {
      synchronized(map) {
        try {
          if (!it.hasNext()) null;
          ret key = it.next();
        } catch (ConcurrentModificationException e) {
          print("Re-iterating");
          it = map.tailMap(key, false).keySet().iterator();
          if (!it.hasNext()) null;
          ret key = it.next(); // Can't throw another exception
        }
      }
    }
  });
}