(4)When a part of a contract which, taken by itself, can and ought to be specifically performed, stands on a separate and independent footing from another part of the same contract which cannot or ought not to be specifically performed, the court may direct specific performance of the former part.Explanation.—For the purposes of this section, a party to a contract shall be deemed to be unable to perform the whole of his part of it if a portion of its subject matter existing at the date of the contract has ceased to exist at the time of its performance.
