// 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) {
        if (_i < l(l)) {
          A a = l.get(_i);
          if (a == iteratorFromFunction_endMarker) fail("no"); // ugly comparison fail-fast redemption
          ret a;
        }
        ret (A) iteratorFromFunction_endMarker; // ugly cast
      }
    }
  });
}

Began life as a copy of #1012173