Assume that jack, hal, and sophia enter into a valid contract for the sale of the restaurant and for the covenant not to compete. part of the agreement says that prior to the closing, jack will have an addition to the dining room built. in order to make sure that jack does this in a timely manner, hal and sophia put a clause into the contract that states that for every day past october 15th that the addition is not finished, hal and sophia are entitled to reduce the purchase price by $500. in order for this clause to be enforceable: