// iterate safely (& quickly) in the face of concurrent modifications
static <A> IterableIterator<A> concurrentlyIterateList(final L<A> l) {
  ret iteratorFromFunction_withEndMarker_f0(new F0<A>() {
    int i;

    A get() {
      int _i = i++;
      synchronized(l) {
        ret _i < l(l) ? l.get(i) : (A) iteratorFromFunction_endMarker; // ugly cast
      }
    }
  });
}