public static <E> ImmutableSet<E> copyOf(Iterable<? extends E> elements) {
    if (elements instanceof ImmutableSet && !(elements instanceof ImmutableSortedSet)) {
      @SuppressWarnings("unchecked") // all supported methods are covariant
      ImmutableSet<E> set = (ImmutableSet<E>) elements;
      return set;
    }
    return copyOf(elements.iterator());
  }

  public static <E> ImmutableSet<E> copyOf(Iterator<? extends E> elements) {