FLOCAS V CARLSON - essential elements: the agreement must be sufficiently certain and intended to be legally binding, the agreement must be one that is not revocable after the death of one of the parties and as is obvious, their agreement cannot prevent a grant of probate of a later will executed in breach of the agreement. Further, the agreement must be specifically enforceable, may be enforced by a third party beneficiary, and if the agreement concerns land it must comply with the Statute of Frauds, unless an exception to the Statute applies.