One of my customers wanted to use Valid Cart for Zen Cart to apply the rules shown in the matrix below. If one of the products in the first column was in the cart, the customer also had to buy one of the products in columns 3-6, otherwise they would not be permitted to check out.
This is easily done with Valid Cart, which will block checkout unless the rule is passed. So trying to checkout with just product 489, for example (which is an add-on product) would not be permitted; the customer would need to add product 334, 385, 361 or 397 to the cart.