Imperative QuickSort

static IEnumerable<T> QuickSort<T>(IList<T> xs)
            where T : IComparable<T>, IEquatable<T>
        {
            var stack = new Stack<Stack<T>>(new[] {new Stack<T>(xs)});            

            while (stack.Any())
            {
                var qs = stack.Pop();

                var head = from q in qs where q.CompareTo(qs.Peek()) == 0 select q;
                var less = from q in qs where q.CompareTo(qs.Peek()) <  0 select q;
                var more = from q in qs where q.CompareTo(qs.Peek()) >  0 select q;

                if (less.Any())
                    stack.Push(new Stack<T>(less));

                if (more.Any())
                {
                    stack.Push(new Stack<T>(head));
                    stack.Push(new Stack<T>(more));
                }
                else
                    foreach (var q in head)
                        yield return q;
            }
        }
    }