When will a prosecutor not express his belief in the credibility of a government witness?

"As a general rule, a prosecutor may not express his ... belief in the credibility of government witnesses. Vouching consists of placing the prestige of the government behind a witness through personal assurances of the witness's veracity, or suggesting that information not presented to the jury supports the witness's testimony." United States v. Necoechea, 986 F.2d 1273, 1276 (9th Cir.1993) (citations omitted). But the prosecutor must have "reasonable latitude in fashioning closing arguments," and can make reasonable inferences based on the evidence. Id.