P was the general contractor for the construction of a housing project for the elderly. P negotiated with D for the supplying of ready-mixed concrete. D would supply all the concrete needed on the project at a price of $13.25 a cubic yard, with deliveries to be made at the times and in the amounts as ordered by P. Things were great until April 1967. A labor dispute disrupted work on the job site. A picket line was maintained on the site until the completion of the project in 1969. For the most part, no deliveries of concrete were made by D, notwithstanding frequent requests. P purchased the balance of its concrete requirements elsewhere. P sued in damages for the additional cost of concrete incurred by virtue of the higher price of the replacement product, as well as the expenses of locating an alternate source. D got the verdict, and P appealed. D claimed impossibility of performance because of the picket line.