How to modify specials to only allow one per customer order?

Jaguar <[email protected]>
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Anybody got any thoughts and logic as to how to accompolish this - 

A modification to either the "Specials" and/or the checkout pages to only allow a customer to only select one pre-defined special from the list of allowed specials for each order.  

Reasoning- specials can be used as promotional items - i.e. "Buy this Car and get these free seat covers or roof rack or pin stripes"  Customers however can only choose one item from the list of specials.

A natural secondary requirement is that if the qualifying product is removed from the cart after the special item has been added that the special would also be removed from the cart so that the customer could not purchase a "special" without the qualifying product.  


This basically means that code needs to exist that will selectively delete specific items from the shopping cart under programatic control based on the criteria of what currently exists in the shopping cart.  

Anybody got any thoughts and logic as to how to accompolish this?
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