A. A contract to make a will or devise or not to revoke a will or devise or to die intestate, if executed after the effective date of this article, may be established only by:
(1) provisions of a will stating material provisions of the contract;
(2) an express reference in a will to a contract and extrinsic evidence proving the terms of the contract; or
(3) a writing signed by the decedent evidencing the contract.
B. The execution of a joint will or of mutual wills does not create a presumption of a contract not to revoke the will or wills.