What is the test for instructing a jury as to the possible motivations of a cooperating witness?

In instructing a jury as to the possible motivations of a cooperating witness, "district courts need only fairly put the issue of a cooperating witness's possible motivations to the jury for its consideration and need not over-emphasize the obvious--that cooperators may have an interest in currying favor with the prosecutor that could affect the substance of their testimony." United States v. Vaughn, 430 F.3d 518, 523-24 (2d Cir. 2005). "As long as district courts intelligibly identify a cooperating witness's possible motivations for the jury's consideration, the cautionary charge given to the jury regarding a cooperating witness's testimony is sufficient." Id.