A court of equity will not decree specific performance where the matter of the contract is such that it cannot supervise or insure its execution.55 This rule is illustrated by the refusal of courts of equity to decree specific performance of contracts involving personal services.56 An injunction may be used to enforce a promise or covenant to forbear. It has been held that where an executory contract contains both positive and negative promises, and the court is unable to enforce the former, it may nevertheless enforce the latter by an injunction. Thus where a professional singer was sued by the proprietor of a theater for specific performance of a contract to sing at his theater upon certain terms, and during a certain period to sing nowhere else, the court refused to enforce so much of the contract as related to the promise to sing, but enforced the promise not to sing elsewhere by granting an injunction.57