Full opinion text
SELYA, Circuit Judge. These appeals, arising out of the drug-trafficking convictions of a dozen New Hampshire residents, suggest that while two New Hampshire men might once have been a match for Satan, see Stephen Vincent Benet, The Devil and Daniel Webster (1937), times have changed. The tale follows. I. BACKGROUND During a two-month trial in the district court, the government mined a golconda of evidence. Because it would serve no useful purpose to recount the occasionally ponderous record in book and verse, we offer instead an overview of the evidence, taken in the light most compatible with the guilty verdicts. See United States v. Ortiz, 966 F.2d 707, 711 (1st Cir.1992), cert. denied, — U.S. -, 113 S.Ct. 1005, 122 L.Ed.2d 154 (1993). Further facts will be added as we discuss specific issues. For almost six years, David Sepulveda conducted an increasingly sophisticated cocaine distribution business in and around Manchester, New Hampshire. Initially, Sepulveda purchased cocaine from a vendor in Nashua, New Hampshire, and transported it to Manchester himself. Over time, Sepulveda expanded his operation, increasing the volume of cocaine and engaging others to handle tasks such as pickup, delivery, and street-level sales. As his enterprise grew more ambitious, Sepulveda began purchasing cocaine from a source in Lawrence, Massachusetts. Faced with the need to retain control while insulating himself from the prying eyes of law enforcement personnel, Sepulveda’s journeys to Lawrence became an elaborate ritual in which he would scrupulously avoid carrying drugs or travelling in the same car with the cocaine that he purchased. On these provisioning trips, Sepulveda was usually accompanied by his brother, Edgar, and a “runner,” that is, an individual who would actually transport the cocaine from Lawrence to Manchester. Frequently, one of Sepulve-da’s distributors or a user in a particular hurry to obtain fresh supplies would join the troupe. Once the cocaine arrived in Manchester, Sepulveda and his associates packaged it in street-level quantities and distributed it to a series of individuals for resale and personal use. The buyers included, among others, defendants Edward W. Welch, Jr., Arline S. Welch, Shane Welch, Kevin Cullinane, Christopher Driesse, Cheryl T. Johnson, Richard E. Labrie, Tony Rood, and William D. Wallace. David Sepulveda made a practice of directing persons who inquired about purchasing small amounts of cocaine to these same individuals. Eventually, David Sepulveda’s reach exceeded his grasp. A federal grand jury indicted him, along with others, for drug trafficking; and, after trial, a petit jury convicted twelve persons, viz., the Sepulveda brothers, the three Welches, Cullinane, Driesse, Johnson, Labrie, Rood, Wallace, and Langlois, on a charge of conspiracy to possess and distribute cocaine. See 21 U.S.C. § 846 (1988). The jury also convicted David Sepulveda on a charge of engaging in a continuing criminal enterprise. See 21 U.S.C. § 848 (1988). Twenty-six appeals ensued. It is no exaggeration to say that the defendants, represented by able counsel, managed to cultivate a profusion of variegated grounds for appeal from the peat of the protracted trial. Because of the sheer bulk and complexity of the proceedings, we issued a special briefing order and then heard oral argument on all twenty-six appeals. We decide today twelve appeals taken by ten defendants (collectively, “the appellants”). After sifting what grains we can locate from the considerable chaff, we conclude that the appellants enjoyed a fair, substantially error-free trial, and that their convictions must stand. In two instances, however, we vacate particular sentences and remand for further proceedings. II. SUFFICIENCY OF THE EVIDENCE Four appellants claim that the evidence is insufficient, as a matter of law, to support their convictions. Because insufficiency claims are commonplace in criminal appeals, the standard of appellate oversight lends itself to rote recitation. Following a guilty verdict, a reviewing court must scrutinize the record, eschewing credibility judgments and drawing all reasonable inferences in favor of the verdict, to ascertain if a rational jury could have found that the government proved each element of the crime beyond a reasonable doubt. See United States v. Echeverri, 982 F.2d 675, 677 (1st Cir.1993); Ortiz, 966 F.2d at 711; United States v. David, 940 F.2d 722, 730 (1st Cir.1991) (collecting cases), cert. denied, — U.S. -, 112 S.Ct. 2301, 119 L.Ed.2d 224 (1992). To sustain a conviction, the court need not conclude that only a guilty verdict appropriately could be reached; it is enough that the finding of guilt draws its essence from a plausible reading of the record. See Echeverri, 982 F.2d at 677; Ortiz, 966 F.2d at 711. Here, the challenged convictions center around a charge of conspiracy to possess and distribute cocaine. To prove a drug conspiracy charge under 21 U.S.C. § 846, the government is obliged to show beyond a reasonable doubt that a conspiracy existed and that a particular defendant agreed to participate in it, intending to commit the underlying substantive offense (here, possession of cocaine with intent to distribute, 21 U.S.C. § 841(a)(1)). See David, 940 F.2d at 735; United States v. Sanchez, 917 F.2d 607, 610 (1st Cir.1990), cert. denied, 499 U.S. 977, 111 S.Ct. 1625, 113 L.Ed.2d 722 (1991); United States v. Rivera-Santiago, 872 F.2d 1073, 1079 (1st Cir.), cert. denied, 492 U.S. 910, 109 S.Ct. 3227, 106 L.Ed.2d 576 (1989). There are no particular formalities that attend this showing: the agreement may be express or tacit and may be proved by direct or circumstantial evidence. See Echeverri, 982 F.2d at 679; Rivera-Santiago, 872 F.2d at 1079. Moreover, in a criminal conspiracy, culpability may be constant though responsibilities are divided; the government does not need to show as a precursor to a finding of guilt that a given defendant took part in all aspects of the conspiracy. See United States v. Benevides, 985 F.2d 629, 633 (1st Cir.1993); United States v. Cruz, 981 F.2d 613, 617 (1st Cir.1992). Using these guideposts, we find that the quantum of evidence presented against each of the four challengers suffices. A. Arline Welch. Four witnesses provided the bulk of the evidence regarding Arline Welch’s role in the conspiracy. Kurt Coriaty testified that he had purchased cocaine from her both in her home and in his, particularly after her husband, Edward Welch, was imprisoned. Coriaty’s partner, Kenneth Milne, stated that Arline Welch gave him cocaine at her home and was present when he purchased cocaine from Edward Welch at the Welch residence. While mere presence is not sufficient to ground criminal charges, a defendant’s presence at the point of a drug sale, taken in the light of attendant circumstances,, can constitute strong evidence of complicity. See Ortiz, 966 F.2d at 711-12. The jury also heard Norberto Perez explain that Arline Welch accompanied David Sepulveda on three buying expeditions to Lawrence, Massachusetts. Perez testified that, in expressing anxiety, she made manifest her awareness of the trips’ purpose, voicing statements like: “Let’s hurry up and get this cocaine so we can get out of here.” Furthermore, Randall Vetrone testified that Arline Welch was present in Edgar Sepulve-da’s apartment while the Sepulveda brothers packaged and sold cocaine. Appellant’s consensual presence in a private home, not her own, while large quantities of drugs were being packaged for resale, possessed eviden-tiary significance. From this fact, coupled with other contextual detail (much of it incul-patory), the jury reasonably could have inferred that she was a member of the ring. See Ortiz, 966 F.2d at 712 (pointing out that criminals rarely seek to expose their felonious activities to innocent outsiders, where such exposure could easily be avoided); United States v. Batista-Polanco, 927 F.2d 14, 18-19 (1st Cir.1991) (to like effect). Keeping in mind the maxim that “criminal juries are not expected to ignore what is perfectly obvious,” Echeverria 982 F.2d at 679, the testimony of these four witnesses and the reasonable inferences to be drawn therefrom formed a serviceable predicate upon which to rest a conviction for conspiracy to distribute cocaine. Accordingly, the district court did not err in denying Arline Welch’s motion for judgment of acquittal. B. Kevin Cullinane. Daniel Santos, a quondam partner of Cullinane’s in the drug distribution trade, testified that Cullinane introduced him to David Sepulveda and that Sepulveda eventually became a principal supplier of cocaine to the Cullinane-Santos partnership. Santos also testified that Cullinane accompanied Se-pulveda on provisioning trips, returning to Santos’s apartment with fresh supplies of contraband. The ubiquitous Norberto Perez corroborated this relationship, testifying that he had travelled to Lawrence on at least five occasions in the company of Cullinane and the Sepulveda siblings in order to replenish cocaine stores. Perez also recreated a conversation that took place between Cullinane and Sepulveda involving the former’s indebtedness to the latter for transactions in cocaine. The government adduced abundant evidence that Cullinane distributed much of the contraband he acquired. Perez and Santos both described Cullinane’s activities as a vendor. John Rice testified that Cullinane delivered cocaine to defendant Christopher Driesse, and that Driesse, in turn, would resell the drugs. Santos confirmed that Cul-linane procured these drugs from David Se-pulveda and that Sepulveda extended credit to Cullinane. Another witness, David Chase, acknowledged that he had purchased up to eight kilograms of cocaine from Cullinane before concluding that, aphorisms about hon- or among thieves notwithstanding, Cullinane could not be trusted. In view of this plenitudinous testimony, the court below appropriately derailed Culli-nane’s quest for acquittal as a matter of law. C. Ernest Langlois. David Sepulveda hired Langlois to be both a drug courier and a torpedo. Lan-glois’s resounding success in the latter role— intimidating Sepulveda’s debtors and, sometimes, his associates—produced a suffusion of testimony limning Langlois’s role in the organization. For instance, David Hill described four occasions on which Langlois used force, or threats of force, to collect debts owed to Sepulveda. Two other witnesses testified that Sepulveda boasted of employing Lan-glois as a strongarm to collect drug debts. Another witness overheard Langlois crowing about the nature of his employment. And no fewer than six witnesses relayed information from which a rational jury could infer that Langlois “rode shotgun” during drug-buying expeditions. As this partial summary indicates, the evidence accumulated against Langlois rose well above the level necessary to sustain the jury’s verdict. D. Cheryl Johnson. Two witnesses, Santos and Kathy Malone (an undercover police officer), testified that David Sepulveda sent them to Cheryl Johnson when they wanted to buy cocaine. Santos said that he purchased cocaine from Johnson on approximately one hundred occasions, during which transactions Johnson offhandedly revealed the full extent of her copious cocaine inventory. Malone stated that she purchased cocaine from Johnson on three occasions. Perez testified that he, too, bought cocaine from Johnson, delivered cocaine to Johnson at Sepulveda’s behest, and accompanied her on at least one buying trip to Lawrence. Although Johnson argues vehemently that the witnesses against her were inherently unreliable, courts must leave such credibility determinations in the jury’s domain. See David, 940 F.2d at 730. Here, the jury was at liberty to credit the testimony, and it, in turn, supplied all the elements necessary to convict. III. SEQUESTRATION Appellants claim that, shortly after sentencing, they learned for the first time that the government housed three key witnesses (Perez, Milne, and Coriaty) in the same cell throughout the trial. Appellants moved for a new trial, alleging that the housing arrangements violated a sequestration order issued by the district court. The government not only contested appellants’ conclusion but also contested the premise on which the conclusion rested, asserting that, though the three men were lodged within the same cell block, they did not share a cell. For reasons that are somewhat opaque, the district court denied the motion without a hearing and without findings anent the accuracy of appellants’ “three to a cell” allegation. Instead, the court determined that, regardless of the dormitory arrangements, its sequestration order had not been flouted. It is against this rather spartan background that we undertake our analysis. A. Rule 615. The sanctum sanctorum of supervised sequestration states in its salient segment: At the request of a party the court shall order witnesses excluded so that they cannot hear the testimony of other witnesses, and it may make the order of its own motion. Fed.R.Evid. 615. The rule more or less codifies common-law sequestration powers, but it is at once less discretionary and less stringent than its forebears. On one hand, the rule cabins the judge’s discretion by affording all parties a right to close the courtroom to prospective witnesses. On the other hand, while the common law supported sequestration beyond the courtroom, see 6 John Wigmore, Evidence § 1840, at 471 n. 7 (1976) (stating that, at common law, the sequestration process involves three parts: preventing prospective witnesses from consulting each other; preventing witnesses from hearing other witnesses testify; and preventing prospective witnesses from consulting witnesses who have already testified), Rule 615 contemplates a smaller reserve; by its terms, courts must “order witnesses excluded” only from the courtroom proper, see Perry v. Leeke, 488 U.S. 272, 281 & n. 4, 109 S.Ct. 594, 600 & n. 4, 102 L.Ed.2d 624 (1989); United States v. Arruda, 715 F.2d 671, 684 (1st Cir.1983). In sum, the rule demarcates a compact procedural heartland, but leaves appreciable room for judicial innovation beyond the perimeters of that which the rule explicitly requires. See United States v. De Jongh, 937 F.2d 1, 3 (1st Cir.1991) (stating that district courts possess “considerable discretion” to fashion orders pertaining to sequestration). Outside of the heartland, the district court may make whatever provisions it deems necessary to manage trials in the interests of justice, see id., including the sequestration of witnesses before, during, and after their testimony, see Geders v. United States, 425 U.S. 80, 87, 96 S.Ct. 1330, 1335, 47 L.Ed.2d 592 (1976), and compelling the parties to present witnesses in a prescribed sequence, see United States v. Machor, 879 F.2d 945, 954 (1st Cir.1989), cert. denied, 493 U.S. 1094, 110 S.Ct. 1167, 107 L.Ed.2d 1070 (1990). Rule 615 neither dictates when and how this case-management power ought to be used nor mandates any specific extra-courtroom prophylaxis, instead leaving the regulation of witness conduct outside the courtroom to the district judge’s discretion. See United States v. Arias-Santana, 964 F.2d 1262, 1266 (1st Cir.1992) (explaining that a federal trial court may enter non-discussion orders at its discretion); see also Arruda, 715 F.2d at 684 (holding that there was “technically” no violation of sequestration where witnesses conversed outside the courtroom). This is not to say, however, that sequestration orders which affect witnesses outside the courtroom are a rarity. As a practical matter, district courts routinely exercise their discretion to augment Rule 615 by instructing witnesses, without making fine spatial distinctions, that they are not to discuss their testimony. Indeed, such non-discussion orders are generally thought to be a standard concomitant of basic sequestration fare, serving to fortify the protections offered by Rule 615. See Perry, 488 U.S. at 281-82, 109 S.Ct. at 600-01. B. Sequestration and Cohabitation. Here, appellants moved in advance of trial for sequestration without indicating to the court what level of restraint they thought appropriate. The court granted the motion in its simplest aspect, directing counsel “to monitor sequestration” and ordering “that witnesses who are subject to [the court’s] order are not to be present in the courtroom at any time prior to their appearance to render testimony.” At trial, the district court expanded its earlier order beyond the Rule 615 minimum, instructing each witness at the close of his or her testimony not to discuss that testimony with any other witness. Appellants accepted the conditions of sequestration described by the court without demurrer. What is more, they did not request any further instructions, say, that witnesses be directed at the end of each day, or before each recess, not to discuss their testimony. Thus, every witness was placed under an order prohibiting discussion of the case with other witnesses only upon the completion of his or her testimony. On these facts, the district court’s denial of relief must be upheld. The court’s basic sequestration order, which ploughed a straight furrow in line with Rule 615 itself, did not extend beyond the courtroom. There has been no intimation that the witnesses transgressed this order. Moreover, because the district court did not promulgate a non-discussion order applicable to any witness until the conclusion of that witness’s testimony, Perez, Milne, and Coriaty were under no obligation, prior to that moment, to refrain from discussing their recollections with each other. Finally, there is no evidence that any of the three ever chatted about the case with another witness after having been admonished to the contrary — or at any earlier time, for that matter. Given this predicate, appellants’ plaint reduces to the unprecedented proposition that witness cohabitation constitutes an automatic violation of a standard sequestration order. The crux of sequestration, however, is communication between witnesses, not shared accommodations or geographic proximity. Social settings, such as communal housing or common work sites, may offer opportunities for witnesses to compare notes and gossip about their testimony, but such environments do not ensure that forbidden conversations will occur. We assume that witnesses, like all other persons subject to court orders, will follow the instructions they receive. Cf. e.g., Richardson v. Marsh, 481 U.S. 200, 206, 107 S.Ct. 1702, 1707, 95 L.Ed.2d 176 (1987) (reiterating the “invariable assumption of the law that jurors follow their instructions”). We conclude, therefore, that the housing arrangement, in and of itself, did not violate an existing sequestration order. If doubt inhered — and we see no room for any — two other considerations would then be decisive. In the first place, a district court’s interpretation of its own order is customarily accorded great weight. See, e.g., Witty v. Dukakis, 3 F.3d 517, 521 (1st Cir.1993); Martha’s Vineyard Scuba Hqtrs., Inc. v. Unidentified, Wrecked & Abandoned Steam Vessel, 833 F.2d 1059, 1066-67 (1st Cir.1987); Lefkowitz v. Fair, 816 F.2d 17, 22-23 (1st Cir.1987). Here, the district court ruled that congregant housing of witnesses did not infringe on the sequestration that it decreed. When a trial court’s interpretation of its own order tracks plain language and the actual sequence of events, that interpretation must be honored on appeal. In the second place, even if some implied ban on congregant housing of prisoner-witnesses existed, breach of a sequestration order would not automatically call for a new trial; rather, the need for a sanction, and the nature of one, if imposable, are matters committed to the trial court’s sound discretion. See United States v. Rossetti, 768 F.2d 12, 16 (1st Cir.1985); Arruda, 715 F.2d at 684. Appellants’ failure to request a broader sequestration order, coupled with the speculative nature of their claim of actual prejudice, renders it impossible to find an abuse of discretion here. See, e.g., Rossetti, 768 F.2d at 16. To sum up, our search of the record in this case discloses no reason to suspect that the government or its witnesses transgressed the existing sequestration order. And as we have indicated, if appellants desired a more vigorous sequestration regime, such as an edict that would have banned cohabitation or other contact amongst prisoner-witnesses, they had a duty to ask for it. They failed to do so. Under these circumstances, the district court appropriately declined to overturn the convictions. See, e.g., De Jongh, 937 F.2d at 3; Rossetti, 768 F.2d at 16; see also Langel v. United States, 451 F.2d 957, 963 (8th Cir.1971) (determining that district court’s refusal to restrict witnesses from communicating with other witnesses, after some had testified, did not constitute error; defendants made “no showing ... that Government witnesses did talk to each other after testifying”). IV. DISPUTES ANENT DISCOVERY Appellants complain vociferously about the government’s conduct during discovery. The complaints have a modest basis in fact. Over the course of this logistically complex trial, the government produced a number of documents in a seemingly lackadaisical fashion. On each of these occasions, defense counsel had to scramble in order to assess the nascent discovery and integrate it into their trial strategy. On other occasions, the government refused to produce documents that defense counsel believed were discoverable either as exculpatory material, see Brady v. Maryland, 373 U.S. 83, 87, 83 S.Ct. 1194, 1196-97, 10 L.Ed.2d 215 (1963), or as fodder for impeachment, see Giglio v. United States, 405 U.S. 150, 154-55, 92 S.Ct. 763, 766, 31 L.Ed.2d 104 (1972); see also 18 U.S.C. § 3500 (1988) (requiring the government to disclose, after direct testimony and on the defendant’s motion, any statement by the witness, in the government’s possession, that relates to the subject matter of the witness’s testimony). We discuss these two species of discovery problems separately. A. Delayed Discovery. Prosecutors have an obligation to furnish exculpatory and impeachment information to the defense in a timely fashion. Although the government’s obligation goes beyond the good-faith requirement of civil discovery, see United States v. Samalot Perez, 767 F.2d 1, 4 (1st Cir.1985), its bounds are not limitless. Patrolling these boundaries is primarily the duty of the nisi prius court. Because the district judge is better attuned to the nuances of the trial, this court must take a deferential view of rulings made in the course of that patrol. When discovery material makes a belated appearance, a criminal defendant must ordinarily seek a continuance if he intends to claim prejudice. A continuance affords time to study the newly emergent information, consider its possible ramifications, change trial strategy (if necessary), assess any potential prejudice, and determine how best to use the information. As a general rule, a defendant who does not request a continuance will not be heard to complain on appeal that he suffered prejudice as a result of late-arriving discovery. See, e.g., United States v. Osorio, 929 F.2d 753, 758 (1st Cir.1991); see also United States v. Diaz-Villa-fane, 874 F.2d 43, 47 (1st Cir.) (concluding, in an analogous context, that a defendant’s cry of unfair surprise “is severely undermined, if not entirely undone, by his neglect to ask the district court for a continuance to meet the claimed exigency”), cert. denied, 493 U.S. 862, 110 S.Ct. 177, 107 L.Ed.2d 133 (1989). Thus, in situations where defense counsel does not seek a continuance upon belated receipt of discoverable information, a court often can assume that counsel did not need more time to incorporate the information into the defense’s game plan. See United States v. Ingraldi, 793 F.2d 408, 413 (1st Cir.1986). This general rule spells defeat for the majority of appellants’ delayed discovery claims. In every instance save one, appellants eschewed a request for a continuance. They have never satisfactorily explained how delays in production caused them any cognizable harm on those several occasions and the record, which reflects that appellants assimilated the new material without any perceptible hitch and used it to good effect, belies any such claim. The lack of demonstrable prejudice sounds the death knell for a “delayed discovery” claim. See United States v. Devin, 918 F.2d 280, 290 (1st Cir.1990) (explaining that a defendant who complains about tardiness in disclosure “cannot rely on whplly conclusory assertions but must bear the burden of producing, at the very least, a prima facie showing of a plausible strategic option which the delay foreclosed”). Hence, we find appellants’ delayed discovery claims, with one exception, to have been waived. The facts referable to the remaining dilatory disclosure claim can be succinctly summarized. Perez not only worked as a courier for David Sepulveda, but also supported his own cocaine habit by peddling drugs. After he was apprehended for selling cocaine, Perez agreed to testify against appellants in return for the United States Attorney’s help in seeking a reduced sentence. Since Perez was the only witness who tied all the defendants to David Sepulveda’s illicit enterprise, his testimony was extremely important to the government’s ease. When defense lawyers began cross-examining Perez, it became apparent that the FBI records furnished in pretrial discovery did not list Perez’s entire repertoire of criminal convictions, especially those stemming from state court proceedings and not reported to the FBI. When Perez acknowledged that a state probation officer had prepared a pre-sentence report for a New Hampshire court, appellants asked for a continuance so that they might obtain this document and more fully research Perez’s criminal history. The district court denied the motion but offered to permit the defense to recall Perez for further cross-questioning should the new information warrant it. The trial continued coincident with the defense’s efforts to secure the presentence report. After some travail, New Hampshire authorities agreed to release the report to the federal court in camera. The district judge found that it contained little fresh material but he nonetheless issued a turnover order. The defense received the report while Perez was still on the witness stand. The judge refused to grant a mistrial or afford appellants any comparable redress. We see no error. The prosecution was caught unawares; it never knew of the report’s existence and, therefore, could not have deliberately withheld it. Furthermore, the rigors of Brady do not usually attach to material outside the federal government’s control — and the presentence report at issue here falls within the scope of this generality. See, e.g., United States v. Aichele, 941 F.2d 761, 764 (9th Cir.1991) (holding that a federal prosecutor had no duty to procure materials prepared for the state courts which were not otherwise under federal control). Last, but far from least, delayed disclosure claims cannot succeed unless the aggrieved defendant demonstrates prejudice arising from the delay. See Devin, 918 F.2d at 290 (refusing to reverse conviction where delayed disclosure of impeachment material “had no effect on the outcome of the trial”); Ingraldi, 793 F.2d at 411-12 (stating that the critical test “is whether defendant’s counsel was prevented by the delay from using the disclosed material effectively in preparing and presenting the defendant’s case”). Here, however, the defense’s delayed receipt of the report did not hinder cross-examination to any appreciable degree. From the start of trial, appellants had a sizeable storehouse of data concerning Perez’s checkered past. In comparison to what was already known, the report, which yielded a relatively inconsequential amount of incremental information, comprised small potatoes. Moreover, the timing of events minimized the possibility of prejudice. Appellants received the report while Perez was still testifying. They were able to incorporate its contents into their cross-examination and employ the information effectively. For aught that appears, the course and outcome of the trial would have been the same no matter when the report surfaced. In our view, the presider’s decision to allow a criminal case to go forward, notwithstanding delayed disclosure of material relevant to impeachment of a witness, should be upheld unless a manifest abuse of discretion looms. See Devin, 918 F.2d at 289. On this record, we can neither criticize the district court’s exercise of its informed discretion nor tamper with the court’s bipartite finding that the government violated no duty and that, in any event, appellants sustained no cognizable prejudice arising out of the presentence report’s belated emergence. B. Denied Discovery. Appellants also complain that the court below, after scrutinizing certain materials in camera, denied their motion to compel discovery. The materials in question consist of various police files, including interview notes. We have reviewed these materials and agree with the lower court that they are outside the purview of the Jencks Act, 18 U.S.C. § 3500, for two reasons. First, to be discoverable under the Jencks Act, a government record of a witness interview must be substantially a verbatim account. See United States v. Newton, 891 F.2d 944, 953-54 (1st Cir.1989). Second, the account must have been signed or otherwise verified by the witness himself. See United States v. Gonzalez-Sanchez, 825 F.2d 572, 586-87 (1st Cir.), cert. denied, 484 U.S. 989, 108 S.Ct. 510, 98 L.Ed.2d 508 (1987). The police files at issue here, including the interview notes, do not meet either of these guidelines and are, therefore, non-discoverable. A fortiori, the district court did not blunder in denying access to them. Y. COCONSPIRATORS’ STATEMENTS During the course of trial, the judge allowed several witnesses to attribute out-of-court statements to one or more declarants, finding, inter alia, that the declarants were coconspirators. Appellants assign error. Although out-of-court statements made by non-testifying declarants ordinarily are excluded as hearsay if offered to prove the truth of the matter asserted, see Fed. R.Evid. 801(c), there are exceptions to the rule. One such exception provides that “a statement by a coconspirator of a party during the course and in furtherance of the conspiracy” is not hearsay. Fed.R.Evid. 801(d)(2)(E). To invoke the exception, a party who wants to introduce a particular statement must show by a preponderance of the evidence that a conspiracy embracing both the declarant and the defendant existed, and that the declarant uttered the statement during and in furtherance of the conspiracy. See Bourjaily v. United States, 483 U.S. 171, 175-76, 107 S.Ct. 2775, 2778-79, 97 L.Ed.2d 144 (1987); Ortiz, 966 F.2d at 714-15. The party at whom the evidence is aimed must object to the statement when it is offered; and, if the district court accepts the evidence de bene, must then ask the court at the close of all the relevant evidence to strike the statement, i.e., to consider whether the proponent fulfilled the requisite foundational requirements by a preponderance of the evidence. See Ortiz, 966 F.2d at 715; United States v. Perkins, 926 F.2d 1271, 1283 (1st Cir.1991); see generally United States v. Ciampaglia, 628 F.2d 632, 638 (1st Cir.), cert. denied, 449 U.S. 956, 101 S.Ct. 365, 66 L.Ed.2d 221 (1980); United States v. Petrozziello, 548 F.2d 20, 23 n. 3 (1st Cir.1977). On five occasions, at least one defendant objected to testimony anent coconspirators’ out-of-court statements. We treat these objections as fully preserved — after all, the district court told counsel that it deemed an objection by one defendant sufficient to preserve the rights of all defendants — and, accordingly, we plumb the record in an effort to determine whether any or all of the district court’s rulings with respect to these statements were clearly erroneous. See United States v. McCarthy, 961 F.2d 972, 977 (1st Cir.1992); United States v. Cresta, 825 F.2d 538, 551 (1st Cir.1987), cert. denied, 486 U.S. 1042, 108 S.Ct. 2033, 100 L.Ed.2d 618 (1988). We begin by considering three pieces of testimony recounting out-of-court statements. The common thread that joins these proffers is that the government adduced some other evidence tending to prove that the declarants were, in fact, coconspirators. We then address two pieces of testimony that are not cushioned in a comparable fashion. A. Supported Statements. 1. Milne Testimony. Milne (a self-confessed coconspirator) served as the wellspring of the first statement. He testified that a defendant, Edward Welch, told him that the police noticed cocaine on his (Welch’s) bed in the course of executing a search warrant. The district court found that the declarant, Welch, was a coconspirator, and that Welch’s statement was made during and in furtherance of the conspiracy. The finding is fully sustainable. Both Coriaty and Milne testified that Welch sold them cocaine he had purchased from David Sepulveda (an arrangement that numerous other witnesses corroborated). This evidence encourages, if it does not demand, the conclusion that Welch worked hand in glove with Sepulveda. A pattern of drug sales between two individuals, looking toward resale to third persons, together with appropriate contextual detail, can support a finding that the two individuals were jointly involved in a drug-trafficking conspiracy. See United States v. Moran, 984 F.2d 1299, 1303 (1st Cir.1993); United States v. Glenn, 828 F.2d 855, 857-58 (1st Cir.1987). The record likewise justifies the conclusion that Welch’s statements to Milne were made during and in furtherance of the conspiracy. We think it is common ground — and common sense — that the reporting of significant events by one eoconspirator to another advances the conspiracy. See United States v. Smith, 833 F.2d 213, 219 (10th Cir.1987). 2. Rice Testimony. Another government witness, John Rice, testified that one defendant, Driesse, mentioned that a second defendant, Rood, sold cocaine for the Sepulveda brothers. The jury found both Driesse and Rood guilty of the conspiracy charge, and the record gives considerable definition to both men’s links to the drug ring. And, moreover, since the sharing of pertinent information about a conspiracy’s mode of operation furthers the conspiratorial ends, see United States v. Munson, 819 F.2d 337, 341 (1st Cir.1987), Driesse’s statement concerning drug sales assisted the charged conspiracy by informing other eoeonspirators of Rood’s role and activities. 3. Malone Testimony. A third statement came in through a police officer, Kathy Malone, who made a number of undercover buys from Sepulveda-supplied vendors. She testified that David Sepulveda’s inamorata, Bambi Burley, told her that she (Bambi) had jilted Sepulveda and asked whether Malone might be “one of those girls that went to New York with him.” Appellants challenge the admission of this statement on the sole ground that Burley, herself, was a stranger to the charged conspiracy. However, this challenge overlooks Perez’s testimony that he collected drug debts for David Sepulveda and delivered the money to Burley. While there is hardly a profusion of evidence depicting Burley as a coconspirator, there is enough to withstand clear error review. B. Unsupported Statements. The two additional instances in which the court below admitted statements under the coeonspirator exception despite contemporaneous objection are qualitatively different. In each instance, there appears to be no record evidence, other than the statement itself, to support its admissibility. One statement arose during the government’s examination of a police detective, Mark Putney. The detective testified that, while executing a search warrant at a dwelling occupied by defendant Cheryl Johnson and her husband, Brian, he answered the telephone: The male caller asked if Brian was home. I stated I was Brian. The caller stated did you pick up the stuff. I said I did. The caller asked if he could come over and pick up a half. I stated sure, come on over. The other statement occurred during direct examination of Joseph Baranski. Bar-anski testified that he sometimes provided transportation for people going to David Se-pulveda’s house and that, on occasion, his passengers would tell him that they were visiting Sepulveda because “they wanted to buy some drugs.” Our review of the record has deterred no extrinsic evidence tending to show that these out-of-court declarants (the unidentified caller to the Johnson residence and the unidentified passengers in Baranski’s vehicle) were involved in the conspiracy, and the government has directed us to no such proof. Following the Supreme Court’s landmark opinion in Bourjaily, and Justice Stevens’s concurrence, 483 U.S. at 185 & n. 2,107 S.Ct. at 2783-84 & n. 2, several of our sister circuits have concluded that the preponderance of evidence required for the introduction of an out-of-court statement under Rule 801(d)(2)(E) must necessarily comprise more than the weight of the statement itself. See, e.g., United States v. Gambino, 926 F.2d 1355, 1361 n. 5 (3d Cir.), cert. denied, — U.S. -, 112 S.Ct. 415, 116 L.Ed.2d 436 (1991); United States v. Garbett, 867 F.2d 1132, 1134 (8th Cir.1989); United States v. Silverman, 861 F.2d 571, 577 (9th Cir.1988); United States v. Zambrano, 841 F.2d 1320, 1344-45 (7th Cir.1988); see also United States v. Daly, 842 F.2d 1380, 1386 (2d Cir.), cert. denied, 488 U.S. 821, 109 S.Ct. 66, 102 L.Ed.2d 43 (1988). We have not yet spoken to the point. See, e.g., United States v. Dworken, 855 F.2d 12, 25 (1st Cir.1988) (deferring resolution). This case presents the issue squarely and requires that we decide it. We hold that a coconspirator’s statement, standing alone, is insufficient to meet the preponderance standard of Rule 801(d)(2)(E). In other words, to satisfy the weight-of-the-evidence criteria for that hearsay exception, there must be some proof aliunde. Though the district court may consider a statement’s contents and the circumstances attending its utterance when gauging the statement’s reliability, see United States v. Gomez-Pabon, 911 F.2d 847, 856 n. 3 (1st Cir.1990), cert. denied, 498 U.S. 1074, 111 S.Ct. 801, 112 L.Ed.2d 862 (1991), admitting the statement into evidence requires some extrinsic proof of the declarant’s involvement in the conspiracy. Thus, because the government developed no independent evidence of who Brian Johnson’s callers or Joseph Baranski’s passengers might have been, or what their status might have been vis-a-vis the charged conspiracy, the statements were improperly admitted under the coconspirator exception to the hearsay rule. There is no bright-line rule for divining when particular errors that result in a jury’s exposure to improper evidence are (or are not) harmless. Rather, a harmlessness determination demands a panoramic, ease-specific inquiry considering, among other things, the centrality of the tainted material, its uniqueness, its prejudicial impact, the uses to which it was put during the trial, the relative strengths of the parties’ eases, and any telltales that furnish clues to the likelihood that the error affected the factfinder’s resolution of a material issue. Gearing our inquiry along these lines, we conclude that the errors in admitting the statements are benign. The telephone talk concerned a peripheral matter, for Brian Johnson was not on trial. Furthermore, several witnesses testified at first hand that his wife and housemate, appellant Cheryl Johnson, trafficked in cocaine. See supra Part 11(D). The passengers’ remarks constituted cumulative evidence. They inculpated only David Sepulve-da, and a googol of witnesses tabbed Se-pulveda as a large-scale narcotics distributor who made countless cocaine sales. Several of these witnesses swore that théy personally purchased drugs from him. Against this tidal wave of evidence, Baranski’s testimony was a drop in the proverbial bucket. Because the record offers every assurance that the errant statements did not affect the trial’s outcome, they were harmless. See United States v. Ladd, 885 F.2d 954, 957-58 (1st Cir.1989); Dworken, 855 F.2d at 26. VI. EXPERT TESTIMONY At trial, the government’s case culminated in the testimony of Commander Richard Gerry of the New Hampshire Drug Task Force. Before Commander Gerry testified, the prosecutor told the court that Gerry’s views would be based upon the trial testimony and his experience as a police officer, and predicted that Gerry would “explain to the jury how the quantities of drugs ... referred to in the ... testimony at trial [were] used and distributed ... from the business aspect.” Based on this representation, the court denied appellants’ motion in limine and permitted the witness to testify. In the initial stages, the testimony went according to plan: Commander Gerry discussed the ways in which drug dealers commonly package their products and reviewed the economics of the cocaine trade (illustrating the profit to be gained by buying and selling in various quantities). Despite this promising start, matters soon deteriorated. Although Commander Gerry offered opinions as to appellants’ roles in the enterprise, characterizing David Sepulveda as “top dog” and the others as rank-and-file members of the organization, cross-examination revealed that these opinions were less the fruit of an expert mind attuned to the testimony in the case than the yield of undifferentiated conversations over the years with unidentified police officers regarding Sepulveda and his associates. So ingrained were the roots of Commander Gerry’s opinions that he was unable to specify the sources of his information or, in the end, articulate a plausible basis for his views. The speculative nature of Gerry’s testimony became starkly apparent when the defense established that he had heard only bits and pieces of the testimony in the case. These rather startling insights cast grave doubt upon both the adequacy of the foundation on which Gerry’s testimony rested and the existence of a fair opportunity for effective cross-questioning. Midway through cross-examination the defense moved to strike the expert’s testimony in toto. The district court granted the motion. We think the defects in the expert’s presentation warranted this step. Fed.R.Evid. 705 provides in pertinent part that an expert witness “may ... be required to disclose [on cross-examination] the underlying facts or data” on which his opinions rest. If cross-examination reveals that the opinions advanced by an expert rest on a wholly inadequate foundation, the judge, on timely motion, may strike the testimony. See, e.g., United States v. Scop, 846 F.2d 135, 142-43 (2d Cir.1988); Benjamin v. Peter’s Farm Condo. Owners Ass’n, 820 F.2d 640, 641 (3d Cir.1987); see also 3 David W. Louisell et al., Federal Evidence § 400, at 709-10 (1979). The district judge also told the jury to disregard the offending testimony “entirely.” His instruction was firm, clear, and to the point. Appellants neither objected to its form nor sought to have the judge improve upon it. The next day, however, appellants moved for a mistrial. The judge denied the motion. On appeal, appellants lament the denial of both their original motion in limine and their subsequent motion for a mistrial. A. The Motion in Limine. The admission of expert testimony is governed by Fed.R.Evid. 702, which authorizes the district court to admit such testimony if, and to the extent that, it will “assist the trier of fact to understand the evidence or to determine a fact in issue....” Id. Because gauging an expert witness’s usefulness is almost always a case-specific inquiry, the law affords trial judges substantial discretion in connection with the admission or exclusion of opinion evidence. See Apostol v. United States, 838 F.2d 595, 599 (1st Cir.1988); United States v. Hoffman, 832 F.2d 1299, 1310 (1st Cir.1987); see also 3 Jack Weinstein & Margaret A. Berger, Weinstein’s Evidence ¶ 702[02] at 702-22 to 702-23 (1993). It follows that a trial judge’s rulings in this sphere should be upheld “unless manifestly erroneous.” Salem v. United States Lines Co., 370 U.S. 31, 35, 82 S.Ct. 1119, 1122, 8 L.Ed.2d 313 (1962); see also Hoffman, 832 F.2d at 1310 (explaining that “the district court’s assessment of what will or will not assist the jury is entitled to considerable deference in the Rule 702 milieu”). Given the government’s preliminary proffer, there was no need for outright exclusion of the anticipated testimony. The Supreme Court has recently reaffirmed that when a party proffers an expert, the trial judge performs a gatekeeping function, determining whether it is reasonably likely that the expert possesses specialized knowledge which will assist the trier better to understand a fact in issue. See Daubert v. Merrell Dow Pharmaceuticals, Inc., — U.S. -, -, 113 S.Ct. 2786, 2796, 125 L.Ed.2d 469 (1993). The witness’s opinions need “not [be] based on first-hand knowledge or observation.” Id. at -, 113 S.Ct. at 2796. Seen in light of these authorities, the district court had a reasonable basis for allowing the expert to testify. If Commander Gerry’s pedagogy proved to be as advertised, his testimony arguably would have assisted the jury in understanding the voluminous evidence that had emerged. Experienced investigators are commonly permitted to testify as experts on topics such as the structure of a criminal enterprise, the economics of the drug trade, and the handling of contraband. See, e.g., United States v. Angiulo, 897 F.2d 1169, 1188-89 (1st Cir.) (allowing veteran FBI agent to offer opinions, based mainly on information presented at trial, about defendants’ roles in gambling ring), cert. denied, 498 U.S. 845, 111 S.Ct. 130, 112 L.Ed.2d 98 (1990); Ladd, 885 F.2d at 959 (allowing experienced police officer to testify about methods of packaging and processing heroin, and relationship to distribution venture); United States v. Angiulo, 847 F.2d 956, 973-75 (1st Cir.) (allowing suitably credentialed agent to offer expert opinions about structure and operation of La Cosa Nostra, including defendants’ relationships to that organization), cert. denied 488 U.S. 928, 109 S.Ct. 314, 102 L.Ed.2d 332 (1988); Hoffman, 832 F.2d at 1310 (allowing knowledgeable federal agent to testify as an expert on the practices and idiom of the cocaine community). In sum, the lower court’s denial of the motion in limine cannot be faulted. The unhappy fact that, in hindsight, the expert turned out to be a dud does not retroactively negate the lawfulness of the court’s original ruling. Trial judges, whose lot is often to make swift battlefield decisions on tangled evidentiary matters, cannot be expected to foretell the future with absolute accuracy. B. The Motion for Mistrial. Appellants’ next assignment of error presents a slightly closer question. Although the district court struck Commander Gerry’s half-completed testimony and told the jurors to disregard what they had heard, appellants assert that the court erred in refusing to grant a mistrial. At the core of appellants’ argument lies their insistence that the judge did no more than hold a farthing candle to the sun; once Gerry’s views were aired, words from the bench, no matter how stentorian the judge’s tone, could not exorcise the resultant prejudice. Granting or denying a motion for a mistrial is a matter committed to the trial court’s discretion. See De Jongh, 937 F.2d at 3; United States v. Chamorro, 687 F.2d 1, 6 (1st Cir.), cert. denied 459 U.S. 1043, 103 S.Ct. 462, 74 L.Ed.2d 613 (1982). The exercise of that discretion always must be informed by the circumstances of the particular case. When, as now, a motion to declare a mistrial has its genesis in a claim that improper evidence came before the jury, the court must first weigh the claim of impropriety and, if that claim is well founded, strike the offending evidence. Next, unless the court believes that the evidence is seriously prejudicial and that a curative instruction will be an insufficient antidote, the court should proceed with the trial after instructing the jury to disregard the evidence. Declaring a mistrial is a last resort, only to be implemented if the taint is ineradicable, that is, only if the trial judge believes that the jury’s exposure to the evidence is likely to prove beyond realistic hope of repair. In this instance, Judge Devine followed the standard paradigm as closely as possible, considering appellants’ delay in offering the mistrial motion. Three factors persuade us that he handled the situation in an appropriate manner. First, courts have long recognized that, within wide margins, the potential for prejudice stemming from improper testimony or comments can be satisfactorily dispelled by appropriate curative instructions. See, e.g., United States v. Figueroa, 900 F.2d 1211, 1216 (8th Cir.), cert. denied 496 U.S. 942, 110 S.Ct. 3228, 110 L.Ed.2d 675 (1990); United States v. Ferreira, 821 F.2d 1, 5-6 (1st Cir.1987); United States v. Cirrincione, 780 F.2d 620, 635 (7th Cir.1985). The instructions given here pass the test of appropriateness; indeed, appellants have not suggested any way in which they might have been improved. Second, Judge Devine did not allow sores to fester. Rather, he halted Commander Gerry’s testimony in midstream and instructed the jurors to discard the faulty evidence. Swiftness in judicial response is an important element in alleviating prejudice once the jury has been exposed to improper testimony. See, e.g., United States v. Pryor, 960 F.2d 1, 2-3 (1st Cir.1992); United States v. Hernandez, 891 F.2d 521, 523 (5th Cir.1989), cert. denied, 495 U.S. 909, 110 S.Ct. 1935, 109 L.Ed.2d 298 (1990). In this case, the judge could scarcely have acted more celeritously. [47] Third, appellate courts inquiring into the effectiveness of a trial judge’s curative instructions should start with a presumption that jurors will follow a direct instruction to disregard matters improvidently brought before them. See United States v. Olano, — U.S. -, -, 113 S.Ct. 1770, 1781, 123 L.Ed.2d 508 (1993); Richardson, 481 U.S. at 206, 107 S.Ct. at 1706-07. Though rebutta-ble, the presumption endures unless it appears probable that, in a particular case, responsible jurors will not be able to put the testimony to one side, and, moreover, that the testimony will likely be seriously prejudicial to the aggrieved party. See United States v. Paiva, 892 F.2d 148, 160 (1st Cir.1989) (collecting cases). Read as a whole, Commander Gerry’s partially completed testimony does not strike us as so compelling that its impact would linger even after the court’s stern admonition. The testimony is virtually indistinguishable from the vast array of other evidence introduced by the prosecution, and, therefore, it is of the cumulative vintage. We have routinely found cumulative evidence impotent when accidentally uncorked. See, e.g., United States v. Ellis, 935 F.2d 385, 393 (1st Cir.), cert. denied, — U.S. -, 112 S.Ct. 201, 116 L.Ed.2d 160 (1991); United States v. Morris, 700 F.2d 427, 431 (1st Cir.), cert. denied, 461 U.S. 947, 103 S.Ct. 2128, 77 L.Ed.2d 1306 (1983). So it is here. Appellants have not successfully rebutted the presumption that the jury heeded the judge’s instructions. In a last-ditch effort to save the day, appellants come at the question of undue prejudice from a slightly more oblique angle. They allege that the government’s questioning of Commander Gerry went, so far beyond the limits of propriety that putting him on the witness stand amounted to prosecutorial misconduct. But the record simply does not support this accusation. The government had a reasonable basis for offering Gerry as an expert witness. Although the decision did not pan out, that is no reason to consign either the prosecutor or the prosecution to the juridical equivalent of philotheoparoptesism. Like judges, prosecutors cannot be held to a standard of utter prescience. For these reasons, we discern no abuse of discretion in either the district court’s initial admission of Commander Gerry’s testimony or the court’s refusal to declare a mistrial after the necessity to strike the testimony arose. In a nutshell, there was no reason to believe that the infelieitously offered evidence remained in the jurors’ minds after it was banished from the case, and, consequently, no nee.d for the court to jettison the baby when the bath water turned tepid. VII. CLOSING ARGUMENT Appellants contend that the prosecutors’ comments during closing argument constituted reversible error because some statements spotlighted appellants’ joint decision not to testify and others unfairly inflamed the jury’s passions. Although these contentions are obviously related, we analyze them separately- A. Comments on Defendants’ Silence. We begin with bedrock. The Fifth Amendment forbids any comment by the prosecutor on the defendant’s exercise of the right to remain silent. See United States v. Robinson, 485 U.S. 25, 30, 108 S.Ct. 864, 868, 99 L.Ed.2d 23 (1988); Griffin v. California, 380 U.S. 609, 615, 85 S.Ct. 1229, 1233, 14 L.Ed.2d 106 (1965). The proposition is more easily stated than applied. There is no bright line marking the precipice between a legitimate assessment of defense witnesses and an impermissible encroachment upon the accused’s silence. Prosecutors who choose to explore such rugged terrain must take particular care not to comment upon, or call the jury’s attention to, a defendant’s failure to take the witness stand. See United States v. Lavoie, 721 F.2d 407, 408 (1st Cir.1983), cert. denied, 465 U.S. 1069, 104 S.Ct. 1424, 79 L.Ed.2d 749 (1984); Rodriguez-Sandoval v. United States, 409 F.2d 529, 531 (1st Cir.1969). Nonetheless, the road runs in both directions, leading to a rough mutuality of obligation. Defense attorneys have a responsibility to exercise reasonable vigilance and direct the trial court’s immediate attention to perceived trespasses. See generally Ortiz, 966 F.2d at 715 (pointing out that “attorneys must usually bear the responsibility for preserving their points”); United States v. Griffin, 818 F.2d 97, 100 (1st Cir.) (discussing the “obligation to alert the district judge to error-in-the-making”), cert. denied, 484 U.S. 844, 108 S.Ct. 137, 98 L.Ed.2d 94 (1987). Although excessive summations may on rare occasions constitute plain error, redressable after the fact notwithstanding the absence of a contemporaneous objection, see, e.g., Arrieta-Agressot v. United States, 3 F.3d 525, 528 (1st Cir.1993), a criminal defendant who believes that a prosecutor’s closing argument goes too far must usually object to the offending statements when and as they are uttered. See id. In this way, the prosecution can clarify ambiguities and correct mislocutions in a timely manner, and, if necessary, the trial judge can administer an immediate antidote, thereby curtailing any damage. None of the appellants chose to testify at trial. Yet, the prosecutors courted trouble in both segments of their closing argument. Initially, one of them asked rhetorically: Did anyone come into this courtroom and say what the Government witnesses told you didn’t happen? Did they? They attacked the witnesses, the DEA, the police officers, [and the government attorneys]. ... On rebuttal, her colleague expanded upon (and twice repeated) the same theme: The United States introduced a lot of evidence during this trial, a lot of facts. And for the most part, there is no evidence in this case to show that what our witnesses said happened did not happen. That is, the defendants have done little or nothing to refute that evidence. Ladies and gentlemen, we stand on the evidence, the overwhelming evidence, the evidence which, for the most part, the defendants have done absolutely nothing to refute.... It was only after the jury had been dismissed for the day that appellants, having sat silently throughout both segments of the prosecutors’ summations, moved for a mistrial based in part on the quoted statements. The trial court denied the motion as untimely and sent the case to the jury the next morning. In the course of the charge, Judge Devine stated on five separate occasions that the government was responsible for carrying the burden of proof, that the defendants had the right to remain silent, and that no inferences might be drawn from the defendants’ election not to testify. In assaying the appropriateness of a prosecutor’s remarks, context frequently determines meaning. See, e.g., United States v. Young, 470 U.S. 1, 11, 105 S.Ct. 1038, 1044, 84 L.Ed.2d 1 (1985); United States v. Akinola, 985 F.2d 1105, 1111 (1st Cir.1993); United States v. Lilly, 983 F.2d 300, 307 (1st Cir.1992). Once the prosecutor’s words are placed in context, we inquire whether “the language used was manifestly intended or was of such character that the jury would naturally and necessarily take it to be a comment on the failure of the accused to testify.” United States v. Glantz, 810 F.2d 316, 322 (1st Cir.) (citations omitted), cert. denied, 482 U.S. 929, 107 S.Ct. 3214, 96 L.Ed.2d 701 (1987); see also Lilly, 983 F.2d at 307. In borderline cases, the standard of review can figure importantly. When no contemporaneous objection appears of rec ord, appellate review is for plain error. See Arrieta-Agressot, 3 F.3d at 528; United States v. Smith, 982 F.2d 681, 682 (1st Cir.1993); see also Fed.R.Crim.P. 52(b). And in the absence of a contemporaneous objection it seems fair to give the arguer the benefit of every, plausible interpretation of her words. See United States v. Donlon, 909 F.2d 650, 656-57 (1st Cir.1990); Glantz, 810 F.2d at 323; cf. Robinson, 485 U.S. at 31, 108 S.Ct. at 868 (noting that counsel’s failure to object contemporaneously suggests that the arguer’s statement is not ambiguous). In this case, the prosecutors’ remarks, taken in context and at face value, do not appear to constitute a comment on the aceuseds’ silence. The government’s closing argument recounted the evidence against each defendant and, while admitting that certain prosecution witnesses possessed unsavory reputations and might profit by cooperation, the prosecutors urged the jury to find that those witnesses testified truthfully. Not surprisingly, defense counsels’ summations played up the credibility theme, systematically besmirching the reliability of the government’s witnesses, stressing internal inconsistencies, and outlining perceived conflicts between the testimony of different witnesses. Throughout, counsel paraded the cooperating witnesses’ criminal records past the jury and made much of what those witnesses stood to gain by currying favor with the authorities. Visualized against this backdrop, and assigning ordinary words their most natural meaning, the prosecution’s argument that the defense had not successfully rebutted incriminating evidence seems not to be a comment on appellants’ failure to testify but a comment about the credibility of the government’s case. Arguments of this stripe do not trespass upon the accused’s right to remain silent. See Lockett v. Ohio, 438 U.S. 586, 595, 98 S.Ct. 2954, 2959, 57 L.Ed.2d 973 (1978) (finding remarks that evidence was “unrefuted” and “uncontradicted” not to violate the Fifth Amendment); see also United States v. Pitre, 960 F.2d 1112, 1124 (2d Cir.1992) (upholding a prosecutor’s comment that defendant had offered no competing explanation); United States v. Castillo, 866 F.2d 1071, 1083 (9th Cir.1988) (uphol