What collateral challenge does a defendant have to a prior conviction for murder?

A collateral challenge to the constitutional validity of a prior conviction--i.e., a claim that, upon entry of a guilty plea to the prior murder charge, the defendant was not admonished and did not knowingly or effectively waive his constitutional "Boykin-Tahl " rights (Boykin v. Alabama, supra, 395 U.S. 238, 89 S.Ct. 1709; In re Tahl, supra, 1 Cal.3d 122, 81 Cal.Rptr. 577, 460 P.2d 449), or a claim that he was under the influence of some substance which rendered his waiver of rights and entry of the plea involuntary and constitutionally defective--presents legal questions of a far different nature than the factual determination of the existence of the prior conviction. Our Legislature has established certain threshold presumptions that apply to such collateral proceedings: the presumption "that official duty has been regularly performed" (Evid.Code, 664); that a court whose judgment is under collateral attack "acted in the lawful exercise of its jurisdiction" (Evid.Code, 666); and the provision that the "burden of producing evidence as to a particular fact is initially on the party with the burden of proof as to that fact." (Evid.Code, 550, subd. (b).)