fact that they are made in a context very different from trial, and therefore are usually irreplaceable as substantive evidence. Under these circumstances, “only clear folly would dictate an across-the-board policy of doing without” such statements. Advisory Committee's Introductory Note on the Hearsay Problem, quoted in Westen, The Future of Confrontation, 77 Mich. L. Rev. 1185, 1193, n. 35 (1979). The admission of co-conspirators’ declarations into evidence thus actually furthers the “Confrontation Clause’s very mission” which is to “advance ‘the accuracy of the truth-determining process in criminal trials.’” Tennessee v. Street, 471 U. S. 409, 415 (1985), quoting Dutton v. Evans, 400 U. S. 74, 89 (1970).
C
There appears to be little, if any, benefit to be accomplished by the Court of Appeals’ unavailability rule. First, if the declarant either is unavailable, or is available and produced by the prosecution, the statements can be introduced anyway. Thus, the unavailability rule cannot be defended as a constitutional “better evidence” rule, because it does not actually serve to exclude anything, unless the prosecution makes the mistake of not producing an otherwise available witness. Cf. Westen, supra; Davenport, The Confrontation Clause and the Co-Conspirator Exception in Criminal Prosecutions: A Functional Analysis, 85 Harv. L. Rev. 1378, 1403 (1972). In this case, for example, out-of-court statements by Michael McKeon and Marianne Lazaro, who testified under immunity, could be introduced based on their testimony in court. The statements of William Levan were admissible because he properly asserted his Fifth Amendment privilege and thereby was unavailable.
Second, an unavailability rule is not likely to produce much testimony that adds anything to the “truth-determining process” over and above what would be produced without such a rule. Dutton, supra, at 89. Some of the available declar-ants already will have been subpoenaed by the prosecution or