What is the burden of proving a defendant's prior conviction in a sexual assault case?

7 In a footnote in People v. Coffey, supra, 67 Cal.2d 204, 60 Cal.Rptr. 457, 430 P.2d 15, it was stated that: "Though the burden of proof as to the constitutionality of the charged prior conviction remains with the prosecution, and the burden of producing evidence rests initially with it, the latter burden shifts to the defendant upon proof of the fact of his having 'suffered' the prior conviction. (See Evid.Code, 550.)" (Id., at p. 217, fn. 15, 60 Cal.Rptr. 457, 430 P.2d 15.)