The following excerpt is from U.S. v. Perez, 144 F.3d 204 (2nd Cir. 1998):
Prosecutors have greater leeway in commenting on the credibility of their witnesses when the defense has attacked that credibility. See, e.g., United States v. Perry 643 F.2d 38, 51 (2d Cir.1981) ("[I]n light of the fact that the defense lawyers attacked the credibility and honesty of the Government's case in their closings, the Government's statements vouching for witnesses were understandable if not laudable.").
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