with whom SUMMERS, J., joins in part, concurring.
11 I concur in today's opinion and write separately to provide an additional analysis of the question to be answered.
12 The manufacturer sued for imposition of products liability must be afforded full opportunity to show that (a) the seat's occupant restraint system was free of the attributed harm-dealing defect and/or (b) the plaintiff's injury is unrelated to the manufacturer's alleged breach of duty. In discharging the onus of showing a Hability-defeating defense, it may likely prove critical also to demonstrate that, at the time in question, the provided restraint mechanism stood disengaged, in whole or in park Due process would indeed mean very little if in the described scenario, a legislatively erected $ 12-420 bar1 were potent enough to destroy the manufacturer's only means of exoneration through its proof of an absent causal nexus between the plaintiff's harm and the allegedly defective product. Oklahoma's due process is co-extensive with that in the federal constitution. Fair School Finance Council v. State, 1987 OK 114, ¶ 54, 746 P.2d 1135, 1148 n. 48. Statutory barriers to essential proof of one's innocence (in a criminal case) or of one's complete exoneration (in a civil case) must yield to the Constitution's superior gauge of fundamental fairness. See, e.g., Rock v. Arkansas, 483 U.S. 44, 62, 107 S.Ct. 2704, 2714, 97 L.Ed.2d 37 (1987); Green v. Georgia, 442 U.S. 95, 97, 99 S.Ct. 2150, 2151-52, 60 L.Ed.2d 738 (1979), Davis v. Alaska, 415 U.S. 308, 319-20, 94 S.Ct. 1105, 1112, 39 L.Ed.2d 347 (1974); Chambers v. Mississippi, 410 U.S. 284, 302, 93 S.Ct. 1038, 1049, 35 L.Ed.2d 297 (1973); Washington v. State of Texas, 388 U.S. 14, 19, 87 S.Ct. 1920, 1923, 18 L.Ed.2d 1019 (1967). Similar law-imposed obstacles to those encountered here in § 12-420, which unreasonably impede full disclosure of the truth, are an anathema to due process. In Re Adoption Of C.D.M., 2001 OK 103, ¶ 1, 39 P.3d 802, 813 n. 5 (Opala, J., dissenting); Three M Investments, Inc. v. Ahrend Co., 1992 OK 33, ¶ 9, 827 P.2d 1324, 1335 n. 34 (Opala, C.J., concurring in part and dissenting in part).
¶ 3 It is far from clear why the question we answer today was certified to this court, unless, of course, counsel succeeded in persuading the federal trial judge that in products litigation the provisions of § 12-420 may be regarded as Oklahoma's substantive law.2 Several extant federal decisions appear to have so concluded with respect to similar enactments from other states.3 Oklahoma's own precedent militates strongly in favor of an opposite view.4 If there was ever any doubt about the character of the § 12-420 bar in its application to products cases, it should now appear more clearly resolved than ever before. Today's opinion plainly declares the bar to apply solely to negligence cases. It does not govern products liability litigation. Our conclusion should firmly free the certifying court of any concern about the bar's application to the trial of this case. Once the § 12-420 bar is swept away, admissibility of an occupant's use or non-use of restraints will, in this case, be controlled solely by the federal evidence law's standards of relevance and, in state-court cases, by the standards of relevance under the Oklahoma Evidence Code.5 They should give ample comfort to the plaintiff and no small amount of protection to the defendant who, much unlike in negligence cases, *211must wage a forensic battle (for its product's reasonable safety) without the benefit of the comparative fault's Hability-reducing defense6 Only then will evenhanded fairness, our legal tradition's age-old prophesy, once again carry the day for the claim we make to uncompromising excellence in our adversarial process of adjudication.
'I 4 In short, for products liability litigation § 12-420 is not entitled to a substantive-legal-norm status.7 This is so because for that class of actions the text of § 12-420, as construed by this court's jurisprudence, leaves entirely unaffected the merits of plaintiffs' claims (and of the defendants' defenses against them).8
T5 For negligence cases, on the other hand, the § 12-420's effect is quite different. There the merits are indeed affected by the impact of the section here in contest, which makes the use (or non-use) of seat belts unavailable both as proof of one's due care as well as of its want.9 In negligence cases, both parties are affected alike by the § 12-420 evidentiary bar. Neither is able to take advantage of the statute that requires seat belts to be fastened. The party plaintiff stands barred from showing compliance (by its use of restraints) and the defendant from demonstrating the plaintiff's nonobservance. On the other hand, in the products lability litigation class, the § 12-420 bar, were it to become applicable, would inure solely to the plaintiff's advantage. There it would most surely exelude a party defendant's tendered proof of an absent causal nexus between its own breach of duty and the plaintiff's harm.
. The provisions of the evidentiary bar in question, found in 47 0.$.2001 § 12-420, are:
Nothing in this act shall be used in any civil proceeding in this state and the use or nonuse of seat belts shall not be submitted into evidence in any civil suit in Oklahoma.
. In the trial of a diversity case U.S. courts are governed solely by federal procedural law. Gasperini v. Center for Humanities, Inc., 518 U.S. 415, 426-27, 116 S.Ct. 2211, 2219, 135 L.Ed.2d 659 (1996); Bankers Trust Co. v. Lee Keeling & Associates, Inc., 20 F.3d 1092, 1099 (10th Cir.1994).
. Gardner By and Through Gardner v. Chrysler Corp., 89 F.3d 729, 736 (10th Cir.1996); Barron v. Ford Motor Co. of Canada Ltd., 965 F.2d 195, 199-200 (7th Cir.1992); Dillinger v. Caterpillar, Inc., 959 F.2d 430 (3rd Cir.1992); Milbrand v. Daimlerchrysler Corp., 105 F.Supp.2d 601, 604-05 (E.D.Tex.2000).
. Bishop v. Takata Corp., 2000 OK 71, 12 P.3d 459, 466.
. 12 O.S.2001 § 2101 et seq.
. Kirkland v. General Motors Corp., 1974 OK 52, ¶ 47, 521 P.2d 1353, 1367.
. See Bishop, supra note 4, at 466, where we held that products liability litigation stands excluded from the purview of § 12-420, explaining that [alt issue in a defective seat products lability case is the condition of the seat belt, and not the conduct of the seat belt user."
. An enactment that does not change the merits of any claim (or of any defenses available against it) but addresses itself solely to matters of evidence, practice or procedure does not add any contents to the corpus of a state's substantive law. Flick v. Crouch, 1967 OK 131, 434 P.2d 256, 261. For definition of an "issue on the merits," see Shamblin v. Beasley, 1998 OK 88, 967 P.2d 1200, 1207.
. In short, in negligence litigation, the use or non-use of restraints is removed from the merits of the case both as an element of the claim and of any available defense against it. See Shamblin, supra note 7 at 1207.