// iterate safely (& quickly) in the face of concurrent modifications
static <A> IterableIterator<A> concurrentlyIterateSet(final NavigableSet<A> set) {
  fO mutex = collectionMutex(set);
  class ConcurrentlyIterateSet extends F0<A> {
    Iterator<A> it = set.iterator();
    A key;
    
    A get() {
      synchronized(mutex) {
        try {
          if (!it.hasNext()) null;
          ret key = it.next();
        } catch (ConcurrentModificationException e) {
          print("Re-iterating");
          it = set.tailSet(key, false).iterator();
          if (!it.hasNext()) null;
          ret key = it.next(); // Can't throw another exception
        }
      }
    }
  }
  ret iteratorFromFunction(new ConcurrentlyIterateSet);
}

Began life as a copy of #1012173