Insofar as the majority affirm the judgment on appeal convicting defendant of the murder of Deputy Sheriff Jack Williams with the personal use of a firearm, I concur. After review, I have found no reversible error bearing on that issue.
In all other respects, I dissent. Less than persuasive is the majority’s analysis as to the rest of the judgment on appeal, defendant’s petitions in habeas corpus, and the People’s petition in mandate. Especially problematic is their discussion of Brown error. (People v. Brown (1985) 40 Cal.3d 512 [220 Cal.Rptr. 637, 709 P.2d 440], revd. on other grounds sub nom. California v. Brown (1987) 479 U.S. 538 [93 L.Ed.2d 934, 941, 107 S.Ct. 837].) More problematic still is their treatment of postjudgment discovery—especially in light of the Los Angeles County jail scandal and the collusion between inmate informants and police and prosecutorial authorities. On both these points, I agree with Justice Broussard’s dissent.
I write separately, however, to address only the question dealing with the elements of the offense of peace officer assault and the special circumstance of peace officer murder.
Defendant was charged with, and convicted of, the first degree murder of Deputy Sheriff Jack Williams, under Penal Code sections 187 and 189, and the peace officer assault of Deputy Sheriff Robert Esquivel, under former Penal Code section 245, subdivision (b) (Stats. 1976, ch. 1139, § 152.5, *1263p. 5106) (hereafter former section 245(b)). It was alleged and found true that in violation of Penal Code section 12022.5, he personally used a firearm in the commission of each offense. It was also alleged and found true, as the sole special circumstance establishing eligibility for the penalty of death, that he committed peace officer murder against Deputy Williams within the meaning of Penal Code section 190.2, subdivision (a)(7) (hereafter section 190.2(a)(7)). He was subsequently sentenced to death.
Defendant now attacks the validity of both the peace-officer-assault conviction and the peace-officer-murder special-circumstance finding. Having considered the matter closely, I am of the view that his attack is successful.
Former section 245(b) provided in relevant part that “Every person who commits an assault with a deadly weapon or instrument or by any means likely to produce great bodily injury upon the person of a peace officer . . . , and who knows or reasonably should know that such victim is a peace officer . . . engaged in the performance of his duties, when such peace officer ... is engaged in the performance of his duties shall be punished by imprisonment in the state prison for” various terms. (Stats. 1976, ch. 1139, § 152.5, p.5106, italics added.)
Section 190.2(a)(7) defines the following as a special circumstance establishing death eligibility: “The victim was a peace officer . . . , who, while engaged in the course of the performance of his or her duties was intentionally killed, and the defendant knew or reasonably should have known that the victim was a peace officer engaged in the performance of his or her duties . . . .” (Italics added.)
Both the offense of peace officer assault and the special circumstance of peace officer murder have as one of their elements that the officer is lawfully engaged in the performance of his duties.
As noted, both the offense and special circumstance contain language defining their scope to situations in which the officer is engaged in the performance of his duties. Both were enacted after our unanimous decision in People v. Curtis (1969) 70 Cal.2d 347 [74 Cal.Rptr. 713, 450 P.2d 33], In Curtis, we expressly recognized that such language had long been construed by the courts of this state to require lawful conduct on the officer’s part. (Id. at pp. 354-356.) And just as expressly, we adhered to that construction. (Ibid.) The basis for this interpretation—which is referred to as the “Curtis rule”—is the premise that an officer cannot be deemed to be engaged in the performance of his duties when he commits an act or omission that is in fact unlawful: his duties, obviously, require lawful conduct and lawful conduct only. (See ibid.) “ ‘[T]he rule of law is well established that where the *1264[legislative body] uses terms already judicially construed, “the presumption is almost irresistible that it used them in the precise and technical sense which had been placed upon them by the courts.” ’ ” (Id. at p. 355, quoting City of Long Beach v. Marshall (1938) 11 Cal.2d 609, 620 [82 P.2d 362], quoting in turn City of Long Beach v. Payne (1935) 3 Cal.2d 184, 191 [44 P.2d 305].) Here, the presumption is in fact “irresistible.”
The majority severely limit the lawfully-engaged-in-duty element and as a consequence greatly expand criminal liability and punishment. Their reasoning is that the Curtis rule is too broad and at its boundary constitutes dictum. They hold that under the rule as redefined, a peace officer is lawfully engaged in the performance of his duties whenever he is “correct[ly] servi[ng]” a “facially valid” warrant—even if his conduct is in fact unlawful. (Maj. opn., ante, at p. 1222.)
The majority err. I cannot agree that the Curtis rule is too broad. In the past 21 years no court has so held; they have all respected stare decisis. It seems eminently sound to declare that a peace officer is not engaged in the performance of his duties when he goes beyond the law. Nor can I agree that at its boundary the rule constitutes dictum. Indeed, in People v. Henderson (1976) 58 Cal.App.3d 349, 353-359 [129 Cal.Rptr. 844], the court clearly, albeit impliedly, held—contrary to the majority’s suggestion—that a peace officer is not lawfully engaged in the performance of his duties, even though correctly serving a facially valid warrant, if his conduct is in fact unlawful. There, the officers in question had correctly executed a facially valid warrant that authorized them to enter and search a residence without first complying with the so-called “knock-notice” requirements of Penal Code section 1531. (See 58 Cal.App.3d at pp. 353-356.) That, however, was simply insufficient. (See id. at p. 358.) Because the so-called “no knock” authorization was determined to be void, an exception to the “knock-notice” requirements was necessary. (Ibid.) The officers’ conduct had to be lawful in actuality in order to be within the performance of their duties. (See ibid.)
For present purposes it is merely idle academic musing as to whether or not the majority’s criticism of the Curtis rule has any theoretical merit. Until today, the rule was firmly established and unquestioned. (See generally 1 Witkin & Epstein, Cal. Criminal Law (2d éd. 1988) Crimes Against the Person, §§ 438, 439, pp. 494-495; id. (1990 pocket supp.) p. 38 [surveying the state of the law on the point].) The majority themselves concede as much, describing the rule as “long-standing.” (Maj. opn., ante, at p. 1217.) It must therefore be presumed—and the presumption is unrebutted—that the rule was recognized by those who drafted the statutory provisions defining the offense of peace officer assault and the special circumstance of *1265peace officer murder. It must also be presumed—and again, the presumption is unrebutted—that the rule was accepted by the drafters and consequently incorporated into their measures. Certainly, if it had in fact been rejected, a clear indication would have been given. But no indication, clear or otherwise, is anywhere apparent.
In any event, the majority’s limitation of the lawfully-engaged-in-duty element and the consequent expansion of criminal liability and punishment—which, in view of the well-settled state of the law, could not have been foreseen—are without effect on acts or omissions, like defendant’s, that were committed before today. “[A]n unforeseeable judicial enlargement of a criminal statute, applied retroactively, operates precisely like an ex post facto law, such as Art. I, § 10, of the Constitution forbids. An ex post facto law has been defined ... as one ‘that makes an action done before the passing of the law, and which was innocent when done, criminal; and punishes such action,’ or ‘that aggravates a crime, or makes it greater than it was, when committed.’ [Citation.] If a state legislature is barred by the Ex Post Facto Clause from passing such a law, it must follow that a State Supreme Court is barred by the Due Process Clause from achieving the same result by judicial construction.” (Bouie v. City of Columbia (1964) 378 U.S. 347, 353-354 [12 L.Ed.2d 894, 899-900, 84 S.Ct. 1697], fn. omitted, italics in original; accord Marks v. United States (1977) 430 U.S. 188, 192 [51 L.Ed.2d 260, 265, 97 S.Ct. 990].)
I turn now to the case at bar. Defendant contends that the instructions given by the court on the offense of peace officer assault and the special circumstance of peace officer murder were erroneous.
In its charge, the court declared: “Every person who commits an assault with a deadly weapon upon the person of a peace officer engaged in the performance of his duties and who knows or reasonably should know that such person is a peace officer and is engaged in the performance of his duties is guilty of a crime.”
The court went on: “If you find the defendant guilty of murder of the first degree, you must then determine if the murder was committed under the following special circumstance, to wit, the killing of a peace officer, a Jack Williams, who, while engaged in the course of the performance of his duties was intentionally killed and said defendant knew or reasonably should have known that such victim, Jack Williams, was a peace officer engaged in the performance of his duties.”
The court concluded: “The phrase ‘in the performance of his duties,’ as used in these instructions, means: [fl] Any lawful act or conduct while *1266engaged in the maintenance of the peace and security of the community or in the investigation or prevention of crime; to wit, the serving of a search warrant.”
It is, of course, error for a court to remove any element of an offense (see, e.g., Sandstrom v. Montana (1979) 442 U.S. 510, 520-523 [61 L.Ed.2d 39, 48-50, 99 S.Ct. 2450]) or a special circumstance (see, e.g., People v. Garcia (1984) 36 Cal.3d 539, 547 [205 Cal.Rptr. 265, 684 P.2d 826]) from the jury’s consideration by means of an instruction that either omits such element entirely or merely contains a mandatory conclusive presumption of its presence.
Defendant claims that by instructing the jury as it did, the court effectively removed the element whether the peace officer in question was lawfully engaged in the performance of his duties.
When we consider a point, such as the present, that turns on the meaning of an instruction defining the elements of an offense (e.g., Sandstrom v. Montana, supra, 442 U.S. at pp. 514, 517 [61 L.Ed.2d at pp. 44-47]) or special circumstance (e.g., People v. Warren (1988) 45 Cal.3d 471, 487 [247 Cal.Rptr. 172, 754 P.2d 218]), the crucial question is, could a juror have reasonably understood the words as the defendant asserts? To my mind, the answer here must be affirmative.
A juror would have reasonably understood the challenged instructions to declare that Deputies Williams and Esquivel and their colleagues were lawfully engaged in the performance of their duties as a matter of law: “The phrase ‘in the performance of his duties,’ as used in these instructions, means: []J] Any lawful act or conduct while engaged in the maintenance of the peace and security of the community or in the investigation or prevention of crime; to wit, the serving of a search warrant.” (Italics added.)
So interpreted, the instructions must be deemed to have raised a mandatory conclusive presumption that the officers were acting lawfully at the time relevant here.
But the issue of the lawfulness vel non of the officers’ conduct was in fact vigorously contested. Specifically, it was disputed whether the officers were acting lawfully when they broke into the Abbey Street residence to execute the warrant to search for narcotics. The People presented evidence and argument to the affirmative. Defendant presented evidence and argument to the negative.
Defendant next contends that the error undermines the validity of the conviction for peace officer assault and the special circumstance finding of peace officer murder.
*1267An instructional error involving the mandatory conclusive presumption of an element does not automatically require the reversal of a conviction (e.g., Rose v. Clark (1986) 478 U.S. 570, 579-582 [92 L.Ed.2d 460, 471-474, 106 S.Ct. 3101]) or the setting aside of a special circumstance finding (see, e.g., People v. Odle (1988) 45 Cal.3d 386, 410-415 [247 Cal.Rptr. 137, 754 P.2d 184]), but is subject to harmless-error analysis under the test of Chapman v. California (1967) 386 U.S. 18 [17 L.Ed.2d 705, 87 S.Ct. 824, 24 A.L.R.3d 1065],
Under Chapman, “before a federal constitutional error can be held harmless, the court must be able to declare a belief that it was harmless beyond a reasonable doubt.” (386 U.S. at p. 24 [17 L.Ed.2d at p. 711].) The “burden of proof” as to prejudice rests on the government. “Certainly error, constitutional error, . . . casts on someone other than the person prejudiced by it a burden to show that it was harmless .... [T]he beneficiary of a constitutional error [is required] to prove beyond a reasonable doubt that the error complained of did not contribute to the verdict obtained.” (Ibid. [17 L.Ed.2d at p. 710].)
As Justice Scalia stated in his concurring opinion in Carella v. California (1989) 491 U.S. 263 [105 L.Ed.2d 218, 109 S.Ct. 2419], “the harmless-error analysis applicable in assessing a mandatory conclusive presumption is wholly unlike the typical form of such analysis. In the usual case the harmlessness determination requires consideration of ‘the trial record as a whole,’ [citation], in order to decide whether the fact supported by improperly admitted evidence was in any event overwhelmingly established by other evidence, [citations]. Such an expansive inquiry would be error here ....
“The Court has disapproved the use of mandatory conclusive presumptions not merely because it ‘ “conflicts] with the overriding presumption of innocence with which the law endows the accused,”’ [citation], but also because it ‘ “invade[s] [the] fact-finding function” which in a criminal case the law assigns solely to the jury,’ [citation]. The constitutional right to a jury trial embodies ‘a profound judgment about the way in which law should be enforced and justice administered.’ [Citation.] It is a structural guarantee that ‘reflects] a fundamental decision about the exercise of official power—a reluctance to entrust plenary powers over the life and liberty of the citizen to one judge or to a group of judges.’ [Citation.] A defendant may assuredly insist upon observance of this guarantee even when the evidence against him is so overwhelming as to establish guilt beyond a reasonable doubt .... In other words, ‘the question is not whether guilt may be spelt out of a record, but whether guilt has been found by a jury according to the procedure and standards appropriate for criminal *1268trials.’ [Citation.] ‘Findings made by a judge cannot cure deficiencies in the jury’s findings as to the guilt or innocence of a defendant resulting from the court’s failure to instruct it to find an element of the crime.’ [Citation.]
“These principles necessarily circumscribe the availability of harmless-error analysis when a jury has been instructed to apply a conclusive presumption. . . . For nothing in [such an] instruction would have directed the jury, or even permitted it, to consider and apply th[e] evidence in reaching its verdict. And the problem would not be cured by an appellate court’s determination that the record evidence unmistakably established guilt, for that would represent a finding of fact by judges, not by a jury .... ‘[T]he error in such a case is that the wrong entity judged the defendant guilty.’ [Citation.]
“Four Members of the Court concluded as much in Connecticut v. Johnson, 460 U.S. 73 (1983) (plurality opinion), which considered whether it could be harmless error to instruct a jury that ‘every person is conclusively presumed to intend the natural and necessary consequences of his act.’ [Citation.] . . . ‘An erroneous presumption on a disputed element of a crime renders irrelevant the evidence on the issue because the jury may have relied upon the presumption rather than upon that evidence. If the jury may have failed to consider evidence of intent, a reviewing court cannot hold that the error did not contribute to the verdict. The fact that the reviewing court may view the evidence of intent as overwhelming is then simply irrelevant. To allow a reviewing court to perform the jury’s function of evaluating the evidence of intent, when the jury never may have performed that function, would give too much weight to society’s interest in punishing the guilty and too little weight to the method by which decisions of guilt are to be made.’ [Citation.] The plurality therefore determined . . . that the use of conclusive presumptions could be harmless error only in those ‘rare situations’ when ‘the reviewing court can be confident that [such an] error did not play any role in the jury’s verdict.’ [Citation.] The opinion mentioned as among those ‘rare situations’ an instruction establishing a conclusive presumption on a charge of which the defendant was acquitted (and not affecting other charges), and an instruction establishing a conclusive presumption with regard to an element of the crime that the defendant in any case admitted. [Citation.]
“Another basis for finding a conclusive-presumption instruction harmless [could be found in the following situation] . . . : When the predicate facts relied upon in the instruction, or other facts necessarily found by the jury, are so closely related to the ultimate fact to be presumed that no rational jury could find those facts without also finding that ultimate fact, making those findings is functionally equivalent to finding the element required to *1269be presumed. The error is harmless because it is ‘beyond a reasonable doubt,’ [citation], that the jury found the facts necessary to support the conviction.” (Carella v. California, supra, 491 U.S. 263, 267-273 [105 L.Ed.2d at pp. 223-227, 109 S.Ct. at pp. 2421-2423], italics in original (conc. opn. of Scalia, J.).)
Applying the principles stated above, I am compelled to conclude that the instructional error here was prejudicial. I simply cannot “declare a belief that it was harmless beyond a reasonable doubt.” (Chapman v. California, supra, 386 U.S. at p. 24 [17 L.Ed.2d at p. 711].) Certainly, the People have not carried their “burden of proof” on the issue.
As stated, the instructions raised an impermissible mandatory conclusive presumption that Deputies Williams and Esquivel and their colleagues were lawfully engaged in the performance of their duties.
This is not one of “those ‘rare situations’ when ‘the reviewing court can be confident that [such an] error did not play any role in the jury’s verdict.’” (Carella v. California, supra, 491 U.S. at p. 270 [105 L.Ed.2d at p. 225, 109 S.Ct. at p. 2423] (conc. opn. of Scalia, J.).)
The “acquittal” rationale does not apply. Obviously, the jury found defendant guilty of the offense of peace officer assault and sustained the special circumstance of peace officer murder.
Nor is the “concession” rationale applicable. As noted, defendant disputed the lawfulness of the officers’ conduct.
Finally, the “necessary implication” rationale does not apply. The jury did not find any facts that logically entailed the conclusion that the officers were lawfully engaged in the performance of their duties.
Accordingly, because the court’s instructions were prejudicially erroneous, I would reverse the conviction for peace officer assault and set aside the special circumstance finding of peace officer murder. Having reversed the conviction, I would vacate the dependent personal-use-of-a-firearm finding. And having set aside the special circumstance finding, which is the sole basis for death eligibility in this case, I would also vacate the sentence of death.
In conclusion, although I concur in affirming the judgment on appeal convicting defendant of the murder of Deputy Williams with the personal use of a firearm, in all other respects I dissent.