Joint or Mutual Wills. A joint or mutual will does not create a presumption of a contract not to revoke the will. A contract to make a will or devise, or not to revoke a will or devise, or to die intestate, can only be established by: (a) Provisions of a will stating material provisions of the contract; (b) An express reference in a will to a contract and extrinsic evidence proving the terms of the contract; or (c) A writing signed by the decedent evidencing the contract. Source: PL 3-106, ch. 3, § 14.