Is it improper for a prosecutor to offer assurances that a witness is credible?

Next, Villareal contends that the prosecutor erred by vouching for the evidence. "It is improper for a prosecutor to offer assurances that a witness is credible or to suggest that evidence available to the government but not before the jury corroborates the testimony of a witness. [Citations.] In either case, prosecutorial comments may be understood by jurors to permit them to avoid independently assessing witness credibility and to rely on the government's view of the evidence." (People v. Cook (2006) 39 Cal.4th 566, 593.) During closing arguments, the prosecutor said, "I'm going to tell you something right now

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