474.155.  Contract to make will or devise, revoke or not revoke will or devise, or to die intestate, how established. — A contract to make a will or devise, to revoke or not to revoke a will or devise, or to die intestate, if executed after January 1, 1981, can 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.  

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The execution of a joint will or mutual wills does not create a presumption of a contract not to revoke the will or wills.