BOUDIN, Circuit Judge.

The issue the parties have framed on this appeal is whether, where the district court adjusts a prior sentence to account for post-sentence cooperation by the defendant, the adjustment may reflect factors other than that cooperation. We sustain the district court's disposition of this case but on a narrower ground, deferring until necessary the larger issue — which may arise only rarely and implicates a difficult and delicate issue of statutory construction.

The defendant, Jonathan Poland, was eighteen years and three months old when he and his seventeen-year old cousin (who was just shy of his eighteenth birthday) made and detonated two pipe bombs during the Spring of 2004. Poland was indicted and convicted of unlawfully possessing an unregistered destructive device, 26 U.S.C. §§ 5861(d), 5841, 5845(f), 5871 (2006), and use of an explosive to maliciously damage or destroy property used in interstate commerce, 18 U.S.C. § 844(i) (2006).

The explosion caused little damage but was close to a truck stop office and fuel of various kinds and so was no minor prank. Poland was sentenced to 63 months imprisonment, the three months over and above the mandatory minimum sentence of 60 months being added because he committed perjury during a suppression hearing. Poland appealed, but later withdrew the appeal and cooperated with the government.

The cooperation led to successful government proceedings against Poland's cousin but, the matter having begun with a juvenile charge, the cousin received a sentence of only three months. After Poland cooperated, the government moved to reduce Poland's sentence to 48 months. Fed.R.Crim.P. 35(b)(1). Poland sought a sentence below 48 months, arguing that all of the ordinary statutory factors could be considered afresh, see 18 U.S.C. § 3553(a), and stressing the much longer sentence he received compared to his cousin.

At a hearing on the motion, the district judge asked the parties to address the impact of new language in Rule 35(b). Under precedents construing former versions, sentencing courts may consider only the extent of the cooperation given in calculating the reduced sentence. The district court noted that both the rule's new language and an advisory committee note suggested that on re-sentencing any of the factors identified in 18 U.S.C. § 3553(a) could now be considered. The pertinent language of the respective changes appears below.

In the end, the district court held that to so read the new rule would work a substantive change and violate the Rules Enabling Act, 28 U.S.C. § 2072(a). United States v. Poland, 533 F.Supp.2d 199, 210 (D.Me. 2008). To avoid this outcome, the district court followed precedent under the old rule and held that "the new Rule 35(b) . . . limit[s] the judge, as before the amendment, to consider only substantial assistance factors in support of the amount of a reduction, notwithstanding the amendment and the Advisory Committee Note." Id. at 211.

The government's recommendation did not bind the district court, which concluded that a reduction to 40 months properly accounted for the cooperation provided by Poland. However, the court said that were it free to consider factors other than cooperation — in particular "the need to avoid unwarranted sentence disparities among defendants with similar records who have been found guilty of similar conduct," 18 U.S.C. § 3553(a)(6) — it would reduce Poland's sentence to 30 months. Id. at 213-14.

Poland now appeals, arguing that under the current Rule 35(b), the judge was free once the motion was made to consider factors other than cooperation in reducing the sentence. We have jurisdiction over the appeal, 28 U.S.C. § 1291; United States v. McAndrews, 12 F.3d 273, 277-78 (1st Cir. 1993); and, as the issue is whether the district court misinterpreted governing law, review is de novo and our authority to provide it is clear despite a circuit split that might matter if the issue were not purely legal.

The merits of the legal issue, to which we now turn, require a precise understanding of how Rule 35(b) evolved. Before the sentencing guidelines, Rule 35(b) permitted a sentencing judge to reduce a sentence, more or less without condition, within 120 days of the sentence or disposition of any appeal. This open ended approach was substantially altered by the Sentencing Reform Act of 1984, Pub.L. No. 98-473, Title II, 98 Stat. 1987 ("the Reform Act"), which adopted a framework for sentencing guidelines that aimed at restricting the discretion of judges in initial sentencing.

Consonantly, the Reform Act limited the judge's authority to alter a final sentence, confining it to three categories, 18 U.S.C. § 3582(c), of which only one directly concerns us. It reads:

The court may not modify a term of imprisonment once it has been imposed except that —

(1) in any case —

. . . .

(B) the court may modify an imposed term of imprisonment to the extent otherwise expressly permitted by statute or by Rule 35 of the Federal Rules of Criminal Procedure. . . .

The Reform Act also rewrote Rule 35(b) to provide:

The court, on motion of the Government, may within one year after the imposition of a sentence, lower a sentence to reflect a defendant's subsequent, substantial assistance in the investigation or prosecution of another person who has committed an offense, in accordance with the guidelines and policy statements issued by the Sentencing Commission. . . .

Reform Act of 1984, § 212(b) (emphasis added).

Before the Reform Act became effective in 1987, Congress again added provisions to both 18 U.S.C. and to Rule 35, which furnished the district court authority, in both initial sentencing and re-sentencing, to impose sentences below prescribed statutory minimums based on a government substantial assistance motion, Anti-Drug Abuse Act of 1986, Pub.L. No. 99-570, Title I, §§ 1007, 1009, 100 Stat. 3207-08 (1986); but we defer discussion of those changes.

Given the underscored language of Rule 35(b) as amended by the 1984 Reform Act — "to reflect a defendant's subsequent, substantial assistance" — other circuit courts (unsurprisingly) concluded that only the defendant's substantial assistance could be considered in granting or measuring a Rule 35(b) reduction. E.g., United States v. Chavarria-Herrara, 15 F.3d 1033, 1037 (11th Cir. 1994). This circuit (and other circuits) similarly construed parallel language in the statute that permits a sentence below a statutory minimum to reflect substantial assistance. E.g., United States v. Ahlers, 305 F.3d 54, 60 (1st Cir. 2002).

In 2002, in re-drafting the rules to improve clarity, the advisory committee proposed a "stylistic" change to Rule 35(b), excising the language that required that a reduced sentence "reflect" a defendant's substantial assistance. Fed.R.Crim.P. 35 advisory committee's note, 2002 amendments. Instead, the 2002 text provided only that "reducing the sentence accord[] with the Sentencing Commission's guidelines and policy statements."

In fact, neither before nor after has the Commission adopted guidelines or policy statements directed to post-sentence reductions based on substantial assistance. There has been from the outset a general guideline provision, not using "reflect" language, for guideline departures based on substantial assistance, seemingly directed to original sentencing. U.S.S.G. § 5K1.1. This circuit, and other courts, read this guideline as implicitly incorporating such a requirement. United States v. Chestna, 962 F.2d 103, 106-07 (1st Cir.), cert. denied 506 U.S. 920, 113 S.Ct. 334, 121 L.Ed.2d 251 (1992).

Despite the 2002 elision, the few circuits to address the issue continued to read Rule 35(b) as limiting post-sentence reductions to assistance-related factors. E.g., United States v. Dobson, No. 07-15732, 2008 WL 4962927 at *2-3 (11th Cir. Nov. 21, 2008) (unpublished); United States v. Lindsay, 254 Fed.Appx. 168, 169-70 (4th Cir. 2007) (unpublished), cert. denied ___ U.S. ___, 128 S.Ct. 1688, 170 L.Ed.2d 382 (2008). Precedent is limited because many circuits limit a defendant's ability to appeal decisions on Rule 35(b) motions. E.g., Haskins, 479 F.3d at 957.

The next step in the chronology was the Supreme Court's 2005 decision in United States v. Booker, 543 U.S. 220, 125 S.Ct. 738, 160 L.Ed.2d 621 (2005), making the guidelines advisory rather than mandatory to save them from constitutional invalidity. Thereafter, apparently believing that Booker required this change, the advisory committee recommended deletion of Rule 35(b)'s then existing requirement that a sentence reduction "accord[] with the Sentencing Commission's Guidelines and policy statements," and the deletion was adopted through the rules process effective December 1, 2007.

Rule 35(b) now reads in relevant part, and so read when the district court considered the government motion in this case, as follows:

Upon the government's motion made within one year of sentencing, the court may reduce a sentence if the defendant, after sentencing, provided substantial assistance in investigating or prosecuting another person.

If this language alone were considered, Poland might well plausibly argue that a re-sentencing court could, in granting a substantial assistance motion, consider other factors and not just substantial assistance in setting the new sentence.

This is so even though the language changes that produced the present rule resulted from two apparent mistakes. The first is the advisory committee's belief that deleting the limiting "reflect" language from Rule 35(b) in 2002 was mere clarification; the second is its view that Booker compelled it to delete in 2007 the remaining reference to the guidelines. The former error is patent, although temporarily masked by the reference to the guidelines, and stands as caution to restyling projects.

As for the second change, the revisors' view (although shared by the Ninth Circuit, United States v. Hicks, 472 F.3d 1167 (9th Cir. 2007)), quite probably misreads Booker. The jury trial concerns that led to invalidating mandatory guidelines for initial sentencing do not bear equally on every regulation of sentencing. Booker's Sixth Amendment rationale built on a tension with jury trial values insofar as the guidelines mandated an increased penalty because of additional conduct ( e.g., using a gun, harming a victim) found only by the judge rather than the jury.

Neither the language of Booker nor its rationale appears to constrain Congress' power to delimit, whether by guideline or otherwise, the conditions on which judges may reduce sentences once imposed; Congress can surely decide whether and when any reductions are allowed and surely as well who should make the downward adjustment and on what basis. Cf. United States v. Fanfan, 558 F.3d 105, 109-10 (1st Cir. 2009) (constraining downward adjustment under 18 U.S.C. § 3582(c) based on revision of crack cocaine guideline).

But despite faulty premises, the 2002 and 2007 changes in Rule 35(b) were adopted through the rulemaking process, and laws based on mistaken premises, whether legal or factual, are not uncommon and are commonly given effect. Cf. Pernell v. Southall Realty, 416 U.S. 363, 94 S.Ct. 1723, 40 L.Ed.2d 198 (1974). So such rule changes arguably control unless barred by the Rules Enabling Act, which famously authorizes "procedural" rules and provides that "[s]uch rules shall not abridge, enlarge or modify any substantive right." 28 U.S.C. § 2072(b).

The district court said that stripping out Congress' "reflect" language from Rule 35(b) and the limiting reference to the guidelines would comprise such a "substantive" change barred by the Rules Enabling Act; thus, the judge read back into the rule the "requirement that the reduction 'reflect' the 'subsequent substantial assistance.'" Poland, 533 F.Supp.2d at 211. This, the judge said, brought back into play earlier case law almost uniformly reading such language to exclude non-as-sistance factors in determining such a reduction.

The Rules Enabling Act's key language is opaque and its few constructions have occurred in quite different contexts. Burlington N. R.R. Co. v. Woods, 480 U.S. 1, 107 S.Ct. 967, 94 L.Ed.2d 1 (1987); Sibbach v. Wilson, 312 U.S. 1, 61 S.Ct. 422, 85 L.Ed. 479 (1941). Among other difficulties, the statute brings into play both an enduring conundrum — the line between substance and procedure — and the puzzle of the statute's reference to "rights." The rules process is among the most important endeavors in the judiciary; any constraining construction of the statute could have far reaching implications.

The sentencing question in our case arises out of a fact pattern that may not be common: usually the only new fact on a Rule 35(b) motion will be the substantial assistance furnished since other factors will have already been fully considered; it is the peculiarity here of the belated sentence for Poland's cousin that created a second argument for a reduction after the original sentence. So we choose to affirm the district court's outcome on a narrower ground and defer for now any encounter with the Rules Enabling Act.

Poland's reduced sentence was below the statutory minimum of sixty months. 18 U.S.C. § 844(i). Had the substantial assistance reduction Poland received occurred at the time of his initial sentencing, it would have been governed by 18 U.S.C. § 3553(e), which was added to the Reform Act by the previously cited Anti-Drug Abuse Act of 1986; the provision states (emphasis added):

Upon motion of the Government, the court shall have the authority to impose a sentence below a level established by statute as a minimum sentence so as to reflect a defendant's substantial assistance in the investigation or prosecution of another person who has committed an offense. Such sentence shall be imposed in accordance with the guidelines and policy statements issued by the Sentencing Commission pursuant to section 994 of title 28, United States Code.

Rule 35(b) similarly provides, in language also added in 1986 by the same statute and now in subsection (4), that on the government's motion based on substantial assistance, "the court may reduce the sentence to a level below the minimum sentence established by statute." Traditionally, the reductions for substantial assistance below the statutory minimum — whether at initial sentencing or on sentence reduction — have been governed by the same standards, see, e.g., United States v. Doe, 940 F.2d 199, 203 n. 7 (7th Cir.), cert. denied 502 U.S. 869, 112 S.Ct. 201, 116 L.Ed.2d 160 (1991), as Congress doubtless intended.

Courts would surely have reached this result in re-sentencing cases, even if Congress had not echoed the "reflects" language in the Reform Act's version of Rule 35(b). Had that phrase been omitted, the courts would still have construed the two enactments in para materia, there being no obvious reason why Congress would want the reflects limitation to apply in one context but not in the other. Cf. In re Sealed Case No. 97-3112 , 181 F.3d 128, 133 (D.C. Cir. 1999) (noting Rule 35(b), § 3553(e), and § 5K1.1 are to be read " in pari materia"). For mandatory minimums, but perhaps only for mandatory minimums, the 2002 deletion of the "reflects" language in the rule may be merely "stylistic."

To put the matter differently, we hold that for re-sentence adjustments that go below the statutory mandatory minimum, Rule 35(b) must be read in conformity with 18 U.S.C. § 3553(e). Congress has never changed the statutory rule that such reductions must reflect only the assistance provided. Nor is there anything in the "legislative history" of changes to Rule 35(b) that reflects any desire to create a discrepancy for post-sentence adjustments below the mandatory minimum.

Poland's statutory mandatory minimum sentence is 60 months. The district court was entitled to reduce the sentence to reflect the assistance provided, which it calculated to justify a sentence of 40 months. It could not, as we read Rule 35(b), provide a greater reduction below the mandatory minimum for any other reason. This disposes of Poland's reliance on comparability with his cousin, any alternative interpretation of U.S.S.G. § 5K1.1 or otherwise pertinent sentencing factors prescribed by Congress.

Affirmed.