code: static <E> ImmutableSet<E> unsafeDelegate(Set<E> delegate) {
      switch (delegate.size()) {
        case 0:
          return of();
        case 1:
          return new SingletonImmutableSet<E>(delegate.iterator().next());
        default:
          return new RegularImmutableSet<E>(delegate);
      }
    }
docs: the elements are known to be non-null.