code: @SuppressWarnings("unchecked")
    static <E> Comparator<? super E> orNaturalOrder(@NullableDecl Comparator<? super E> comparator) {
      if (comparator != null) { // can't use ? : because of javac bug 5080917
        return comparator;
      }
      return (Comparator<E>) Ordering.natural();
    }
docs: Returns the specified comparator if not null; otherwise returns {@code Ordering.natural()}.
  This method is an abomination of generics; the only purpose of this method is to contain the
  ugly type-casting in one place.