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[[18727]]Commercial impracticability under the UCC is basically a codification of the common law doctrine of impossibility of performance, which, in turn, is grounded in the doctrines of mutual mistake and frustration of purpose. The purpose of each of the defenses is to… discharge an obligor from his duty to perform a contract where a failure of a basic assumption of the parties produces a grave failure of the equivalence of value of the exchange to the parties. And all (of the defenses) are qualified by the same notions of risk assumption and allocation. Aluminum Co. of America v. Essex Group, Inc., 499 F. ...