Wholesaler-Retailer interaction

Consider the following interaction sequence between retailer and wholesaler. The retailer first requests the price of some quantity of the product. It then requests a sale, paying the required amount, and gets as a return a set of items. Let us examine the sell operation. Its spec, based on the type model shown in Figure 4.6, could be... 

This spec says nothing about how the values of p, q, and payment are related to any attributes of the retailer. Nor does it say what effect the returned set of items has on the retailer. Although this lack of detail is useful when you re designing the wholesaler in isolation, in the bigger picture of the overall interaction between retailer and wholesaler, those details are important. 

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