Is a defendant deprived of a fair trial when government witnesses are invited to comment on his credibility and candor, and when he in turn is asked to comment on their truthfulness?

QUESTIONS PRESENTED 1. Is a defendant deprived of a fair trial when government witnesses are invited to comment on his credibility and candor, and when he in turn is asked to comment on their truthfulness? 2. Where an uncalled witness in a foreign country is a long-term government informant with whom the government was still in contact at the time of trial, and where defense counsel was unable to contact or speak to him, is it error for the trial court to instruct the jury that the witness is equally unavailable to both sides? -i