An individual hires a contractor to build a house, with the express order to use a specific type of electrical wiring. the contractor's crew, however, does not use the electrical wiring requested, but rather, another brand. the error is not discovered until after the house is almost complete, and it is too late for the contractor to replace the wiring without rebuilding the entire structure. the prospective homeowner does not want to pay, but a court rules that despite the mistake, the contractor has discharged the obligation under the contract. which type of discharge is present?