42. Injunction to perform negative agreement.- Notwithstanding anything contained in clause (e) of section 41, where a contract comprises an affirmative agreement to do a certain act, coupled with a negative agreement, express or implied, not to do a certain act, the circumstance that the court is unable to compel specific performance of the affirmative agreement shall not preclude it from granting an injunction to perform the negative agreement: Provided that the plaintiff has not failed to perform the contract so far as it is binding on him.