class ListNode is
	var value : int;

	var prev : ListNode;
	var next : ListNode;
	var head : ListNode;

	fun GetNext() : ListNode is
		if next != head then
			return next;
		else
			return nil;
		end
	end

	fun GetPrev() : ListNode is
		if prev != head then
			return next;
		else
			return nil;
		end
	end

	fun GetValue() : int is
		return value;
	end
end

class List is
	var head : ListNode;

	fun Init() is
		head = new ListNode();
		head.prev = head;
		head.next = head;
	end

	fun Append(data : int) is
		var n : ListNode;
		var ntl : ListNode;

		ntl = head.prev;

		n = new ListNode();
		n.value = data;

		n.prev = ntl;
		n.next = head;
		n.head = head;

		ntl.next = n;
		head.prev = n;
	end

	fun GetFirst() : ListNode is
		return head.next;
	end
end

class ListDemo is
	fun Main() is
		var list : List;

		list = new List();
		list.Init();

		list.Append(5);
		list.Append(6);
		list.Append(7);
		list.Append(8);

		var n : ListNode;

		n = list.GetFirst();
		while n != nil do
			Builtin.WriteLine(n.value);
			n = n.GetNext();
		end
	end
end