Permutations - writing LISP in C#

Permute.cs

    public static class Extensions
    {
        public static IEnumerable<IEnumerable<T>> Permute<T>(this IEnumerable<T> items)
        {
            var isEmpty = true;
            foreach (var item in items)
            {
                isEmpty = false;
                foreach (var p in items.Except(item.Yield()).Permute())
                    yield return item.Yield().Concat(p);
            }

            if (isEmpty)
                yield return Enumerable.Empty<T>();
        }

        public static IEnumerable<T> Yield<T>(this T item)
        {
            yield return item;
        }
    }