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'Commercial impracticability' under Section 400.2-615 RSMo 1969 (U.C.C.), which section reads, in part: 'Excuse by failure of presupposed conditions Except so far as a seller may have assumed a greater obligation and subject to the preceding section on substituted performance: (a) Delay in delivery or nondelivery in whole or in part by a seller who complies with paragraphs (b) and (c) is not a breach of his duty under a contract for sale if performance as agreed has been made impracticable by the occurrence of a contingency the nonoccurrence of which was a basic assumption on which the contract was made or by compliance ...