SPOTTSWOOD W. ROBINSON, III, Circuit Judge:
This appeal tenders for resolution questions as to the examinatorial entitlements of the criminally accused at federal preliminary hearings. Appellants, Lawrence D. Coleman, Jorge D. Dancis and Ronald Shepard, were arrested and charged 1 with the commission of unrelated crimes within the District of Columbia. Following arrest, each was brought before a judicial officer2 for the proceedings prescribed by then Rule 5 of the Federal Rules of Criminal Procedure.3 Coleman and Dancis each sought, and each was denied, a subpoena requiring the attendance at his preliminary hearing of the only apparent eyewitness to his alleged offenses. Shepard, during his preliminary hearing, was restricted in cross-examination of the complainant and a corroborating Government witness, and in the presentation of evidence of his own.
Subsequent to the preliminary hearings, the three appellants joined in a class-action complaint in the District Court. They sought declaratory judgments that the preliminary hearings were defective, writs of mandamus reopening them, and an injunction restraining, pendente lite, presentation of their cases for grand jury consideration.4 The District Court denied a preliminary injunction and dismissed [1193]*1193the action,5 and this appeal ensued. For reasons which follow, we reverse the District Court's judgment to the extent that it denied a declaration that Dancis’ preliminary hearing was faulty and remand the case in order that the declaration may be made. In all other respects we affirm, but without prejudice to rectification in the criminal proceeding pending against Dancis of the error committed at his preliminary hearing.
Some of the questions advanced on appeal are common to the cases of two or more of the three appellants.6 Each appeal, however, also tenders an issue not present in either of the others. We therefore treat the three cases separately.
I. COLEMAN’S APPEAL
After joining in this appeal, Coleman was indicted in two bills for multiple violations of the federal narcotic laws.7 Two days before oral argument on the appeal, he entered a plea of guilty to two counts, one in each of the two indictments.8 He insists that his preliminary hearing, at which the charges laid in one of the indictments were aired, was fatally infirm and that we should now direct that it be reopened.9 He further argues, as he must, that the plea does not stand in the way of the appeal brought here for that purpose. We do not agree.
A plea of guilty consummating a voluntary and intelligent choice of available alternatives10 has serious ramifications for the criminal proceeding. It operates as an admission of all material facts alleged in the count or counts pleaded to,11 and thus dispenses with the need to prove them.12 More important, however, is the effect of the plea beyond this service for the Government. “[T]he plea is more than an admission of past conduct; it is the defendant’s consent that judgment of conviction may be entered without a trial — a waiver of his right to trial before a jury or a judge.”13 It is thus an act bringing [1194]*1194the prosecution to a successful end by solemnly establishing the offense so acknowledged :
A plea of guilty differs in purpose and effect from a mere admission or an extra-judicial confession; it is itself a conviction. Like a verdict of a jury it is conclusive. More is not required; the court has nothing to do but give judgment and sentence.14
We speak, of course, only of a valid plea of guilty, which by definition is one which is understandably as well as voluntarily made.15 The plea cannot take on that character unless, among other things, the accused is aware of the consequences,16 of which its conclusivity on the proceeding is not the least. But if, on the other hand, the accused possesses that awareness and nonetheless enters the plea, it is truly an “intentional relinquishment or abandonment”17 of defenses and procedural entitlements of which the defendant otherwise might have availed himself.18
So it is that “[a]n unqualified plea of guilty, legitimately obtained and still in force, bars further consideration of all but the most fundamental premises for the conviction.”19 Emerging from the numerous decisions on the subject is the well settled rule that an unconditional plea of guilty waives all prior infirmities in the prosecution which affect neither the court’s jurisdiction nor the substantive sufficiency of the indictment.20 The decisions make equally plain the corollary that as long as the plea stands, it bans consideration of other types of defects on appeal.21 [1195]*1195We are unable to distinguish, in terms of the bar, a direct appeal in the criminal proceeding from an appeal in collateral litigation designed, as here, to secure benefits at a criminal trial.
As we have said, the key to waiver, and to conclusivity of the plea on further litigation, combines the accused’s understanding that that would become a consequence of pleading guilty and his willingness to so plead on those term.22 The plea is invalid, of course, if the accused did not realize that it would sacrifice his right to judicial determination of the factual and legal issues by a trial with ultimate appellate review.23 And surely an accused is not barred from pursuing his appeal by pleading guilty where a statute authorizes him to do both.24 But there is an inherent conflict in the notion that, absent such a statute — the case in the federal system —an accused can intelligently waive adverse pre-plea rulings with a view to reposing the litigation and still keep the issues alive on appeal.
We are mindful of the suggestion that the ends of sound judicial administration might be better served by permitting the accused to preserve adverse rulings on legal questions for appeal without the burden of a potentially futile trial on the merits.25 That position was asserted in United States v. Doyle,26 but the court answered:
The premise is sound enough but the conclusion does not follow. There are a number of ways to deal sensibly with such a case without departing from the [conclusivity rule]. A plea expressly reserving the point accepted by the court with the Government’s consent or a stipulation that the facts are as charged in the indictment are two; failing either of these, the defendant can simply stand on his not guilty plea and put the Government to its proof without developing a case of his own.27
We agree, and believe too that without a choice between the competing values by an exercise of statutory or rule-making powers,28 the availability of appeal must depend on the breadth of the accused’s waiver.29
That Coleman’s waiver was broad enough to encompass his present appeal is a matter not fairly open to doubt on the record. Unlike the two counts to which he pleaded guilty, other counts of the indictments against him charged crimes for which there were mandatory penalties,30 which anyone would naturally wish to avoid. The Government was willing to accept Coleman’s plea on the two counts carrying lighter penalties, but only if the plea were tendered right away. The Govern-. [1196]*1196ment made it plain that it would not hold its offer of acceptance open just to enable Coleman to litigate this appeal. This was fully appreciated by defense counsel, who protested the Government’s position to the District Judge, and by Coleman himself, who acknowledged to the judge his understanding that the plea would foreclose both trial and appeal.31 We conclude that Coleman’s outstanding plea of guilty has precisely that effect.
The plea, we are also persuaded, was unaffected by Coleman’s simultaneous protestation of innocence.32 While pleas of guilty are usually accompanied by express admissions of guilt,33 lack of that element in no wisé conditions the plea.34 In North Carolina v. Alford,35 the Supreme Court held flatly that “[a]n individual accused of crime may voluntarily, knowingly' and understandably consent to the imposition of a prison sentence even if he is unwilling or unable to admit his participation in the acts constituting the crime.”36 Long prior to the decision in Alford, penalties imposed on defendants who maintained even vigorously that they did not commit the charged offenses, but who nonetheless entered pleas of nolo contendere, were upheld against constitutional attacks upon them.37 In Alford, the Court perceived no difference between a guilty plea refusing to avow the commission of the offense and one simultaneously maintaining the accused’s innocence.38 As long as each is voluntarily and intelligently entered, there is no cause for constitutional complaint.39
Similarly, the Government’s refusal to extend its acceptance of the plea for the period Coleman continued in this litigation did not vitiate the plea.40 The Government’s declination stemmed from its concern that its case against Coleman might deteriorate in the meantime, and it is not for us to substitute our judgment for its. Nor is this a “situation [in which] the prosecutor or judge, or both, deliberately employ their charging and sentencing powers to induce a particular defendant to tender a plea of guilty.” 41 On the contrary, the record supports amply the conclusion that Coleman made his plea voluntarily, and knowingly at the expense of the appeal. We hold that his attack on his preliminary hearing is now barred.
II. SHEPARD’S APPEAL
Appellant Shepard was charged with assaulting a Deputy United States Marshal42 while a prisoner in the [1197]*1197cellblock43 of the District of Columbia Court of General Sessions.44 A judge of that court, sitting as a committing magistrate,45 presided over his preliminary hearing. The complaining witness, Deputy Marshal John H. Lonien, testified that while he was on duty in the cellblock, Shepard committed an unprovoked attack upon him, striking him above the right eye with a fist. Another Government witness, Herbert Rutherford, employed as a guard in the cell-block, corroborated Marshal Lonien’s testimony.
Shepard’s counsel was permitted considerable latitude in cross-examination of these witnesses as to matters they had testified to on direct examination. The judge, however, sustained the Government’s objections to a number of inquiries directed to them on other topics. The specific complaint Shepard refers to us runs to the judge’s rulings on eleven questions propounded to Marshal Lonien and four to Guard Rutherford.46 Those questions, in the main, solicited testimony as to disparaging remarks assertedly directed to cellblock personnel by prisoners other than Shepard, and to the nature and extent of any injuries inflicted by Shepard on Marshal Lonien and of injuries allegedly sustained by Shepard himself.47 The judge also ruled out Shepard’s proffer of photographs purporting to show his post-altercation physical condition, and inquiry of a defense witness as to whether cellblock personnel had tried to confiscate the photographs.
The more common basis of the Government’s objections to defense counsel’s cross-examinatorial approach was that he was venturing beyond the boundaries of a hearing designed to explore probable cause and embarking on a quest for discovery of elements of the Government’s case.48 After some amount of prior ambivalence on the subject, Shep[1198]*1198ard now disclaims any attempt at discovery, as distinguished from refutation of probable cause.49 - He further argues that the questions addressed to Marshal Lonien and Guard Rutherford bore a substantial relationship to the existence or nonexistence of probable cause.
At the outset, we put aside an otherwise litigable circumstance which has come to our attention. Exercising our power to judicially notice proceedings in related cases,50 we learned that, after oral argument and submission in this court, Shepard entered a plea of guilty to a lesser offense included within the charge on which he was indicted. Unlike their stance with respect to Coleman, appellees have not contended that Shepard’s plea affects his appeal, nor indeed did either side see fit to even make us aware of the plea. We have pointed out that a plea of guilty operates as a barrier only when it possesses the elements of a valid waiver of further litigation.51 In Coleman’s instance, the record satisfies us that his plea did,52 but in Shepard’s we have no record on the matter at all.53 Accordingly, we do not consider this potential problem.
A. Discovery at Preliminary Hearings
Former Rule 5(c) granted the accused, and its present counterpart continues to confer, the right to “cross-examine witnesses against him” at a preliminary hearing.54 The true dimension of that right is bound to depend in considerable measure upon the degree to which discovery by the defense may be a purpose the preliminary hearing is designed to serve. That,’ in turn, is a topic upon which the judges of this court have expressed views which, to say the least, have not been entirely harmonious.55 One view has been that the sole objective of a preliminary hear[1199]*1199ing is to determine whether there is probable cause to believe that the accused has committed an offense, and that the accused may lay claim to the benefit of only so much discovery as may become incidental to a properly conducted inquiry into probable cause.56 That view has now been incorporated into federal jurisprudence by the Federal Magistrates Act.57
This Act provides mandatorily, with exceptions later to be considered,58 for “a preliminary [hearing] ... to determine whether there is probable cause to believe that an offense has been committed and that the arrested person has committed it.”59 The reason the Act indulges the preliminary hearing no independent discovery role is evident from its legislative history. During hearings before the Senate Committee on the Judiciary, witnesses urged “that preliminary examination afforded a necessary and useful medium for defense counsel to obtain discovery of the prosecution’s evidence.”60 The Committee, however, was “of the opinion that the problem of discovery should be treated separately from that of the preliminary hearing.” 61 Although the need for expanded pretrial discovery procedures was recognized,62 the Committee felt that
The preliminary hearing does not present an ideal opportunity for discovery. It is designed for another purpose; namely, that of determining whether there is probable cause to justify further proceedings against an arrested person. Thus, the degree of discovery obtained in a preliminary hearing will vary depending upon how much evidence the presiding judicial officer thinks is necessary to establish probable cause in a particular case. This may be quite a bit, or it may be very little, but in either event it need not be all the evidence within the possession of the Government that should be subject to discovery.63
The Committee accordingly concluded64 “that discovery procedure should remain separate and distinct from the preliminary examination. . . . ”65
That settles the matter, of course, for Shepard and others whose hearings took place after the effective date of the Act.66 The mission of the [1200]*1200hearing is an investigation into probable cause for further proceedings against the accused. It does not include discovery for the sake of discovery. To be sure, the evidence the Government offers to establish probable cause is by nature also discovery for the accused. So also is information adduced on cross-examination of Government witnesses on the aspects of direct-examination testimony tending to build up probable cause. In those senses, some discovery becomes a by-product of the process of demonstrating probable cause. But in no sense is discovery a legitimate end unto itself.
B. Cross-Examination at Preliminary Hearings
To say merely that discovery is not a primary function of federal preliminary hearings is to respond only incompletely to the issue Shepard poses. As we have said, former Rule 5(e) conferred upon the accused the right to “cross-examine witnesses against him,” 67 and that right he continues to enjoy.68 Moreover, in Coleman v. Alabama,69 the Supreme Court, in holding that a preliminary hearing to ascertain probable cause to bind an accused for additional proceedings is a critical stage of the criminal process at which the Sixth Amendment right to counsel obtains,70 pointed out as one of the considerations supporting its holding that “the lawyer’s skilled examination and cross-examination of witnesses may expose fatal weaknesses in the [prosecution’s] case that may lead the magistrate to refuse to bind the accused over.”71 Since the right to counsel is the right to effective assistance of counsel,72 Coleman requires us to evaluate Shepard’s challenge with the increased solicitude appropriate when constitutional rights are at stake.73 This we have done, and we are led to the conclusion that the District Court’s disposition of Shepard’s grievance should not be disturbed.
According to Shepard’s brief on appeal, the purpose of his counsel’s questions on cross-examination of the two Government witnesses was to show that “(a) there were no physical injuries to the Marshals; (b) there were severe injuries to Mr. Shepard rendering him unconscious; (c) the assault charge was brought as a subterfuge for the Marshals’ own conduct; (d) the Marshals were provoked by disparaging [1201]*1201remarks by prisoners other than Mr. Shepard; (e) there was mass confusion in the cellblock seriously impeding the perception of the Marshals; and (f) there was evidence that Mr. Shepard acted in self-defense, if he acted at all.” 74 The first difficulty we have encountered is that the handling of the cross-examination made this understanding all too difficult to come by. Cross-examination at a preliminary hearing, like the hearing itself, is confined by the principle that a probe into probable cause is the end and aim of the proceeding,75 and the line between refutation of probable cause and discovery into the prosecution’s case ofttimes is thin. Here counsel’s purpose in pro pounding the questions which the presiding judge excluded was unquestionably blurred by the fact that counsel frequently appeared to be off on an impermissible quest for discovery. At no time prior to the rulings complained of did counsel delineate for the judge’s edification the factual thesis he was seeking to promote. Only as the hearing neared its close, and after the rulings had been made, did counsel broach anything remotely similar to the defensive theory now explained on appeal.76 Our reading of the hearing record leaves us with the conviction that the presiding judge, when ruling on counsel’s questions, could hardly divine what counsel had in mind. Therefore, we cannot say that he committed error in barring responses to inquiries that seemed unrelated to the task of evaluating probable cause.
Moreover, cross-examination is properly to be limited at preliminary hearing, as at trial, to the scope of the witness’ direct examination. To the extent that it is not — and here it was not —cross-examination ostensibly, even if undesignedly, becomes an effort at some sort of discovery. We do not suggest that magistrates may not indulge variations from the usual order of offering evidence, and during presentation of the Government’s case permit the defense to get in elements of its own. But when cross-examination exceeds the range of direct examination unaccompanied by an elucidation of its connection with probable cause, it is small wonder that discovery is taken to be the examiner’s goal.
An even more important consideration stems from the difference between the objective of the preliminary hearing and that of the trial. While, of course, conviction necessitates proof at trial of all elements of a crime beyond a reasonable doubt, it suffices for purposes of a binding over for trial that the evidence show “probable cause to believe that an offense has been committed and that the defendant has committed it.” 77 The preliminary hearing is not a mini-trial of the issue of guilt, but is rather an investigation into the reasonableness of the bases for the charge, and examination of witnesses thereat does not enjoy the breadth it commands at trial.78 “A preliminary hearing,” the Supreme Court has said, “is ordinarily a much less searching exploration into the merits of a case than a trial, simply because its function is the more limited one of determining whether probable cause exists to hold the accused for trial.” 79
It is the contrast of probable cause and proof beyond a reasonable [1202]*1202doubt that inevitably makes for examinatorial differences between the preliminary hearing and the trial. Probable cause signifies evidence sufficient to cause a person of ordinary prudence and caution to conscientiously entertain a reasonable belief of the accused’s guilt.80 Proof beyond a reasonable doubt, on the other hand, connotes evidence strong enough to create an abiding conviction of guilt to a moral certainty.81 The gap between these two concepts is broad. A magistrate may become satisfied about probable cause on much less than he would need to be convinced. Since he does not sit to pass on guilt or innocence, he could legitimately find probable cause while personally entertaining some reservations. By the same token, a showing of probable cause may stop considerably short of proof beyond a reasonable doubt, and evidence that leaves some doubt may yet demonstrate probable cause. In the instance before us, the testimony of two witnesses on direct examination furnished more than an ample foundation for a finding of probable cause which the cross-examination allowed did not impair. By our appraisal the convoluted defensive theory Shepard now says he wanted to develop 82 was not likely to change the result. Whatever its potency as a basis for a reasonable doubt at trial, its capability to dissolve enough of the Government’s showing to negate probable cause strikes us as highly improbable. We speak not only of the cross-examination which was banned but also of the items of similar purport which on Shepard’s presentation were excluded.83 In any event, the situation is far too cloudy to warrant a grant of the extraordinary relief which Shepard seeks.
Magistrates presiding over preliminary hearings, no less than judges presiding over trials, are endowed with broad powers to supervise examination of witnesses.84 Beyond that, they should be indulged some leeway in their resolution of probable cause issues. Courts should not upset these judgmatic exercises unless a supervisory excess or a decisional error is clearly shown,85 and we do not perceive either here. Shepard’s counsel was permitted to cross-examine each Government witness closely as to the elements of his direct testimony and, for the reasons stated, we cannot say that disallowance of the questions ruled out was improper. For similar reasons, we are unable to say that the photographs and the questions as to the defense witness possessed such a tendency to dissolve probable cause that their exclusion was erroneous. A writ of mandamus lies only to enforce a plain, positive duty; it is not available to exact a response to a dubious claim.86 At best, any obligation to reverse the rulings on the excluded evidence is entirely too unclear. We accordingly affirm as to Shepard.
III. DANCIS’ APPEAL
Dancis, our third appellant, was charged with two violations of the Marijuana Tax Act.87 The charges came on for ventilation at a preliminary hearing [1203]*1203over which a United States Magistrate presided. The magistrate denied his counsel’s request for a subpoena requiring the attendance of an unnamed undercover agent, who apparently was the sole available eyewitness to the two marijuana transactions attributed to Daneis. The Government’s only witness at the hearing was the agent’s supervisor, whose testimony as to the alleged transactions was necessarily hearsay, and as to the transactor’s identity was simply that the agent had identified Daneis from a six-year old photograph. The magistrate, on a finding of probable cause, held Daneis for grand jury action, and the District Court, in the case under review, held that the hearing was legally sufficient.
Daneis argues that each of two flaws vitiated his preliminary hearing. One is that the magistrate’s refusal to allow him access to the undercover agent’s testimony was prejudicial error. The other is that the Confrontation Clause 88 outlaws the magistrate’s finding of probable cause solely upon the hearsay testimony of the agent’s supervisor.89 We deem it unnecessary to reach the constitutional issue posed by Daneis’ second contention90 because we agree that he is on sound ground in advancing the first.91
A. Defensive Evidence at Preliminary Hearings
Former Rule 5(c) confirmed the right of an accused to “introduce ev[1204]*1204idence in his own behalf” at his preliminary hearing.92 It also imposed the requirement that an affirmative decision on probable cause be reached “on the evidence.”93 The specifications of present Rule 5.1(a) are identical.94 Thus a federal preliminary hearing is not only the occasion upon which the Government must justify continued detention by a showing of probable cause, but also an opportunity for the accused to rebut that showing.95 Rule 5(c) made it clear that it is as much the arrestee’s prerogative to endeavor to minimize probable cause as it is the Government’s to undertake to maximize it, and that both sides must be indulged reasonably in their respective efforts. And the Government’s demonstration on probable cause must surmount not only difficulties of its own but also any attack the accused may be able to mount against it.96
In sum, “the evidence” which alone must guide resolution of the probable cause issue is the whole evidence— for the defense as well as for the prosecution. The magistrate must “listen to [the] versions [of all witnesses] and observe their demeanor and provide an opportunity to defense counsel to explore their account on cross-examination.” 97 The magistrate “sits as a judicial officer to sift all the evidence before resolving the probable cause issue. . . . ”98 He “cannot decline to issue subpoenas on the ground that only the Government’s evidence is probative.”99
These provisions of the Rules and our interpretations of them are now reinforced by the holding in Coleman v. Alabama100 that the Sixth Amendment secures for the accused the assistance of counsel at a preliminary hearing having for its purpose a determination on probable cause to hold him for further proceedings.101 Among counsel’s potential contributions, the Court stated, is “skilled examination . of witnesses [which] may expose fatal weaknesses in the [prosecution’s] case that may lead the magistrate to refuse to bind the accused over.” 102 It cannot be gainsaid that what the Sixth Amendment mandated for Alabama’s preliminary hearing it exacts equally for the federal preliminary hearing which, we repeat, is exclusively an exploration into probable cause to hold the accused to answer the prosecution further.103 Nor can it be doubted that [1205]*1205Coleman demands more than the mere presence of counsel at the hearing. The right to counsel which Coleman declared would amount to no more than a pious overture unless it is a right to counsel able to function efficaciously in his client’s behalf. The Sixth Amendment’s guaranty of counsel is a pledge of effective assistance by counsel,104 and Coleman makes it clear that federal preliminary hearings, as critical stages of criminal prosecutions, require no less. If the accused’s counsel is reduced to a state of impotence in the discharge of this responsibility, it is evident that the accused is deprived of the very benefit which the Sixth Amendment’s boon of counsel was designed to confer.105
So, an accused is normally entitled to subpoenas compelling the attendance at his preliminary hearing of witnesses whose testimony promises appreciable assistance on the issue of probable cause.106 The test, our past utterances on the subject have indicated, couples the witness’ materiality107 with an absence of good cause for not requiring his presence,108 and its operation does not depend upon which side might have been expected to call the witness.109 Certainly an accused will not in every instance qualify for a subpoena for the production of a Government witness at his preliminary hearing, but where he succeeds in a plausible showing that that witness could contribute significantly to the accuracy of the probable cause determination, the request for the subpoena should be granted. “This,” we have said, “is consistent with the principal purpose of the preliminary hearing as a mechanism to determine whether the evidence is adequate to establish probable cause.” 110
We think the testimony of the undercover agent Dancis desired at his preliminary hearing met the standard of materiality. From aught that appears, he was the only available person who could testify to the two charged marijuana transfers from personal observation, and by the same token the only one who could directly identify the party responsible for them. Since probable cause to bind Dancis over for further prosecution depended on the caliber of the Government’s showing that he was that party and that what he did on the two occasions under scrutiny was illegal, it seems clear that the witness he requested could have given testimony bearing critically upon those matters. In Washington v. Clemmer111 it was the complainant in a rape case who was sought, [1206]*1206and in Ross v. Sirica 112 the only three eyewitnesses to a murder. In both cases we held that denial of the accused’s access to them was error,113 and it appears to us that the sole eyewitness to the transgressions laid to Dancis was equally material.
As we admonished in Ross, “[w]hatever the full reach of the accused’s subpoena rights at a preliminary hearing, . . . he is entitled to compel the attendance of eyewitnesses unless, of course, 'because of physical or psychological disability in a particular case’ such witnesses cannot attend.”114 That seems the more so when the nature of the Government’s presentation at Dancis’ preliminary hearing is taken fully into account. The Government offered but one witness, and he could testify on the vital issues of offenses and identity only from hearsay, and it is evident that that weakened the showing. To the extent that hearsay is employed, the effort to establish probable cause becomes more prone to attack since the reliability of the absent hearsay declarant always becomes an added factor to be reckoned with. In Ross, where, similarly to Dancis’ case the Government’s one witness at a preliminary hearing on a murder charge was a police officer who could merely relay what three eyewitnesses had told him about the crime, two judges of this court aptly observed, without dispute from the rest, that
A judicial officer engaged in a judicial determination of probable cause can hardly rest easy solely with the hearsay account of the policeman of what these eyewitnesses told him if the eyewitnesses can be available, so that he can listen to their versions and observe their demeanor, and provide an opportunity to defense counsel to explore their account on cross-examination. The presence of those witnesses impresses us as falling within the orbit of the rights conferred upon the accused by the fourth sentence of Rule 5(c). . . . 115
Indeed, the problem addressed in Ross is compounded in the situation before us now. The Government’s evidence at Dancis’ preliminary hearing was not only hearsay but also hearsay without any apparent means of refutation whatever. The undercover agent was not only absent from the hearing but at the time was also totally unidentified. He did not sign the complaint against Dancis, nor was he named in it, and the testimony at the hearing referred to him simply by his code name “John P.” Defense counsel’s inquiries on cross-examination as to his real name, and even as to generic characteristics,116 drew objections from the Government which the magistrate sustained. There was little or nothing in the Government’s .presentation to lend credit to the reliability of either the agent or the observations purportedly incriminating and identifying Dancis. It is difficult to imagine a case wherein the accused was more helpless to defend against a hearsay attribution of probable cause.
To say, as we do, that the testimony of the absent witness was material does not mean necessarily that the refusal of [1207]*1207the subpoena was error vitiating the preliminary hearing. A refusal may be justified, and if it is a finding of probable cause climaxing the hearing must stand.117 The record before us, however, is singularly devoid of any such justification. There is no hint that the undercover officer was physically unamenable to a subpoena or in any way disabled from responding to it.118 There is no suggestion that his information about the episodes under exploration was to any extent privileged from compulsory disclosure.119 Nor is there a basis for attributing the denial of the subpoena to the exigencies of any undercover operation.120 The magistrate did not predicate the denial upon any of these grounds, nor did the Government even urge any of them. And to the extent that the record may furnish indications that the magistrate was satisfied on probable cause without hearing from the undercover agent,121 it suffices to repeat that the issue thereon cannot properly be resolved without accommodating reasonable demands of the prosecution and the defense for the production of evidence capable of shaping the outcome.122
B. Return of Indictment
Concluding as we do that the magistrate erred in refusing Daneis the benefit of the undercover officer’s testimony, we are left to determine how the mistake should be corrected. The first question confronting that effort is whether the indictment returned against Daneis forecloses rectification of the error. It is well settled that an indictment itself establishes sufficient proba-' ble cause for holding the accused for trial,123 and that explains why we have consistently held that he is not entitled to a preliminary hearing where he is indicted before a hearing is held. Typical situations are those wherein the accused is indicted prior to arrest on the charges,124 or prior to the date set for preliminary hearing on the charges,125 or where he is in custody on another charge when indicted.126 In none of these instances can a preliminary hearing serve a need to probe probable cause to detain the accused, for the indictment has fulfilled that need.127 Nor can the hearing be invoked merely as a device for obtaining discovery, for discovery independent of the ascertainment of [1208]*1208probable cause is not one of its functions.128
The case before us, however, is markedly different. Dancis was given a preliminary hearing on the pending charges prior to return of the indictment. At that hearing, by his allegation, he was denied the right then conferred by former Rule 5(c) to examine a witness in his own behalf. That contention, we think, keeps Dancis in court despite the post-hearing rendition of the indictment.
Where in terms of the requirements of the Rules, a preliminary hearing is defective, the accused is obviously entitled to a reopening of the hearing to enable remediation of the defect if that can be done before a grand jury acts in the matter. “To say that the preliminary hearing[] [was] defective is to say that the determination of probable cause was inadequate and should not operate to deprive the accused of his liberty pending grand jury consideration.”129 Accordingly, we have not hesitated to direct a supplemental preliminary hearing on demonstration that at the original hearing the accused did not get his just due.130 That remedy,' of course, is additional to the accused’s prerogative to seek a dissolution of the commitment itself.131
We have, moreover, adhered to the view that such a deprivation can be remedied by a reopening of the hearing even after return of an indictment. The indictment bars relitigation of the question of probable cause, of course, but the fact remains that save for disrespect of the accused’s rights he would have enlarged his insight into the Government’s case against him — as a by-product of efforts pro and con on probable cause in the course of the Government’s submission. Since, however, a Rule 5(c) right was dishonored, the accused lost a part of that by-product, and we have held that the hearing should be reopened to afford the accused an opportunity to retrieve the part which has been lost.132
That is the rationale of our past decisions respecting post-indictment supplemental preliminary hearings. The supplemental hearing was not an occasion for a reexamination of probable cause, for the indictment had settled that issue. Nor was its purpose discovery for its own sake, but only such discovery as would inexorably have accompanied scrupulous observance of the Rule 5(c) right. Reopening of the preliminary hearing, then, emerged as both an appropriate sanction for and remediation of Rule 5(e) violations, not unlike, though less drastic than, the more familiar sanction of former Rule 5(a) which the Supreme Court’s Mallory decision supplied.133 The effect of the supplemental hearing was to confer upon the accused the benefit of only so much incidental discovery as would have been his but for the infringement of Rule 5(c).134
Since our development of this prophylactic doctrine, however, the Federal [1209]*1209Magistrates Act135 was adopted, and the question becomes whether the Act has set the doctrine for naught. The Act lays down the broad mandate that an arrestee be offered a preliminary hearing,136 the date for which must be fixed at his initial appearance before a judge or magistrate after his arrest.137 The hearing must be conducted “within a reasonable time following initial appearance, but in any event not later than” ten days where the arrestee remains in custody138 or twenty days where he is enlarged in the interim.139 The Act specifies, however, in Section 303(e) that “[n]o preliminary examination . . . shall be required to be accorded an arrested person if at any time subsequent to the initial appearance of such person before a judge or magistrate and prior to the date fixed for the preliminary examination ... an indictment is returned. ...”140
We need not ponder whether the Act erects a barrier to a post-indictment pretrial reopening of a preliminary hearing which is seriously defective under the criteria specified in the Rules. To be sure, the remedy we provided in Ross v. Sirica,141 in circumstances comparable to those here, was a remand to the magistrate for a supplemental hearing.142 But Ross, like other decisions in its day, rested on the theory that discovery was as much a function of the preliminary hearing as an investigation into probable cause to bind the accused for further prosecution.143 The Act was passed after Ross was decided, and whether or not it imposed an absolute ban on reopenings of preliminary hearings, it certainly destroyed discovery as a reason for ordering a reopening.144 Furthermore, nothing in Ross nor in any of our other holdings inexorably required a supplemental hearing when another procedure might do equal or superior service. On the contrary, it was our uniform practice to relegate the problem of remediation, not to the magistrate, but to the trial judge when the deficiency in the preliminary hearing first came to light after the accused had been convicted.145
Even more importantly, Ross predated not only the Act but also the Supreme Court’s decision in Coleman v. Alabama,146 and Coleman introduced an element of the remedy problem which this court has not hitherto had- occasion to consider. To put in a nutshell what we later elucidate, Coleman identifies the constitutional right to effective assistance of counsel as an incident of preliminary hearings and assures yindiea[1210]*1210tion of that right.147 It is evident, then, that the return of an indictment against the accused cannot eliminate the need for procedures of some kind to redress violations of the Coleman right.148 On the other hand, the Coleman-type remedy, when invoked prior to trial, embraces an array of alternatives for dissipating potential prejudice.149 So, even if a reopening of the hearing survives the Act as one of these alternatives, Dancis cannot compel that particular choice.
Mandamus is an extraordinary remedy,150 available only to enforce a clear, unequivocal duty.151 It does not lie to control an exercise of judgment.152 Dancis, we say, is entitled, notwithstanding the indictment, to protection against injury resulting from the magistrate’s mistake.153 But Dancis is not necessarily entitled to receive that protection through a supplemental hearing before the magistrate, assuming even that the Act leaves that course open as a possibility. Additionally, as we later discuss, a supplemental hearing suffers by comparison with other alternatives open in this case.154 It is unnecessary, then, for us to decide whether in any event such a hearing would be available after an indictment has issued, and that decision we pretermit to another day.
C. The Remedy
Coleman v. Alabama155 bestowed upon Dancis an inviolable right to protection against harmful consequences flowing from the magistrate’s refusal to permit examination of the undercover agent at the preliminary hearing.156 Coleman also offers guidance as to the proper course to be taken where, following trial and conviction, the accused seeks to remedy a constitutional deficiency in his preliminary hearing. In Coleman, the Court could not determine from the record whether the absence of counsel from the accused’s preliminary hearing actually worked prejudice at his trial, so it vacated the conviction and remanded the case to the Alabama courts for an inquiry on that score.157 In the event that prejudice was found, the remedy would be a new trial; but if, applying the test set out in Chapman v. California,158 it was found that the in[1211]*1211firmity in the preliminary hearing was harmless, the conviction would be reinstated.159 Even prior to Coleman, as we have said, in our own post-conviction decisions we have settled upon the same remedial course where serious flaws in preliminary hearings left them infirm.160
These decisions chart the route to be traveled when the error is judicially detected only after conviction. But that is not Dancis’ situation, for while he has been indicted he has not yet been tried. Dancis could, of course, proceed to trial secure in the knowledge that should he encounter prejudice because of the refusal of the subpoena requested at his preliminary hearing, any conviction would have to be set for naught. But it would be senseless to require him to undergo trial and conviction before undertaking to repair the defect. On the contrary, we have emphasized that one deprived of his just due at a preliminary hearing not only may, but ordinarily must, seek rectification in court immediately.161 This position may reflect the undesirability of permitting an accused to withhold his complaint until after he has lost a gamble on the jury’s verdict. In any event, it certainly reflects a decided preference to avert prejudice before it occurs rather than to wait and deal with it after the fact.
We think, too, that the problem of remediation arising pretrial, no less than when it emerges after conviction, is one to be addressed by the court itself. There is nothing to be gained, over and above handling by the judge, by sending the case back to the magistrate for a supplemental preliminary hearing after the accused has been indicted. The magistrate cannot reinvestigate probable cause, for the indictment establishes its existence, nor is there anything else that officer can do that the judge cannot do better. The concern is that the accused may suffer prejudice at his forthcoming trial by the infringement of his rights at the hearing, and avoidance of prejudice is the function and duty of the judge himself. Obviously the judge could not, by directing the magistrate to reopen the preliminary hearing, shift his own responsibility to the magistrate. In our view, where an unmitigated blunder at a preliminary hearing may infect the ensuing trial, the court is obligated to scrutinize the accused’s claim of possible injury, and to take appropriate corrective action.
We perceive no reason why, in the circumstances presented, the problem at hand cannot be fully accommodated within the framework of' the criminal prosecution pending against Dancis. The judge who is to preside at Dancis’ trial is amply equipped to dissipate whatever risk of prejudice was bred by Dancis’ inability to call the undercover officer as a witness at his preliminary hearing. There are opportunities uniquely open to the trial judge, particularly with the cooperation of the parties, to seek remediation of the magistrate’s error before any real damage is done, and the judge is free to draw upon his imagination in the kind of remedy-fashioning which traditionally has been a prerogative of the federal judiciary. Moreover, it would be advantageous to the prosecution and the accused alike to eliminate the problem from the realm of future litigation.
The indictment against Dancis named the undercover agent, and it may well be [1212]*1212that he testified before the grand jury. If so, the trial judge might consider making the agent’s grand jury testimony available to defense counsel. Alternatively, since after indictment the Government no longer has had an interest in keeping the agent’s identity secret, and obviously will have to produce him as a witness at trial, a voluntary interview may be indicated. If need be, the judge might set appropriate bounds for an interview and arrange for the agent’s participation therein. It may be that, by consent of the parties and approval of the judge, a deposition by written interrogatories to the agent would suffice as an expedient. Without any effort to exhaust the possibilities, we make these observations simply as suggestions of procedures calculated to safeguard Dancis against prejudice consistently with an orderly and expeditious progression of the case to trial. The point we do emphasize is that the judge is in a position to do anything the magistrate could now undertake, and indeed to do a great deal more. We leave for the trial judge, in the first instance, the decision on suitable relief.
The judgment appealed from is reversed insofar as it denied the requested declaration that Dancis’ preliminary hearing was rendered defective by the magistrate’s refusal to allow a subpoena commanding the appearance of the undercover agent as a witness, and the case is remanded to the District Court in order that such a declaration may be made.162 In all other respects, the judgment is affirmed, but without prejudice to steps in the criminal proceeding pending against Dancis which are designed to appropriately remedy the error committed at his preliminary hearing.
So ordered.