stack = {count: 0, head: none};

fun push(stack, value) 
{
	node = {prev: none, item: value};

	node.prev = stack.head;
	stack.head = node;
	stack.count = stack.count + 1;
}

fun pop(stack)
{
	if stack.head == none:
		return none; # return it; ?

	value = stack.head.item;
	stack.head = stack.head.prev;
	stack.count = stack.count - 1;

	return value;
}

push(stack, 1);
push(stack, 2);
push(stack, 3);
x = pop(stack);
y = pop(stack);
z = pop(stack);
w = pop(stack);

assert(x == 3);
assert(y == 2);
assert(z == 1);
assert(w == none);