CSScheme.Primitive.isList C# (CSharp) Метод

        static bool isList(Object x)
        {
            Object slow = x, fast = x;
            for(;;)
            {
                if (fast == null) return true;
                if (slow == rest(fast) || !(fast is Pair)
                    || !(slow is Pair)) return false;
                slow = rest(slow);
                fast = rest(fast);
                if (fast == null) return true;
                if (!(fast is Pair)) return false;
                fast = rest(fast);
            }
        }