In Galmish v. Cicchini, 90 Ohio St.3d 22, 27, 2000 Ohio 7, 734 N.E.2d 782, the Ohio Supreme Court stressed that the point is not "'how the agreement is to be proved, because as a matter of law the writing is the agreement. Extrinsic evidence is excluded because it cannot serve to prove what the agreement was, this being determined as a matter of law to be the writing itself. The rule comes into operation when there is a single and final memorial of the understanding of the parties. When that takes place, prior and contemporaneous negotiations, oral or written, are excluded; or, as it is sometimes said, the written memorial supersedes these prior or contemporaneous negotiations.'" Id., quoting from In re Gaines Estate (1940), 15 Cal.2d 255, 264-265, 100 P.2d 1055, 1060.