In Estate of Smith (1948) 31 Cal.2d 563, the decedent had written a holographic revocatory clause across the face of a copy of an attested will (the original of which had been left with the decedent's attorney). The Supreme Court held the decedent had incorporated the attested will in the holographic clause, and said (p. 567): " It has long been settled in this state that either a holographic or an attested testamentary instrument may refer to and incorporate another testamentary instrument executed with different statutory formalities or an informal or unattested document, so long as the reference is unmistakable or with the aid of extrinsic proof can be made so." (See 7 Witkin, Summary of Cal. Law (8th ed. 1974) Wills and Probate, 143, p. 5660.) "The papers incorporated by reference are used to construe and apply the will, and do not become part of the will in the same sense as those integrated. Hence the holographic will may be regarded as entirely in the testator's handwriting, as required by the statute." (7 Witkin, supra, p. 5661; Estate of Smith, supra, 31 Cal.2d 563, 567-568.)