Defendants' burden on the reformation question is a heavy one. Defendants must establish by clear and convincing evidence that the instrument does not express the intent of the parties.In re Vernon Hills, Inc., 348 F.2d 4, 9 (7th Cir. 1965). See also Aetna Insurance Co. v. Paddock, 301 F.2d 807 (5th Cir. 1962); Timber Investors, Inc. v. United States, 587 F.2d 472 (Ct.Cl. 1978).