Does a prosecutor have to avoid indirect reference to the failure of a defendant to testify?

The prosecutor, who would argue the merits of the government's case, must sometimes walk a narrow line to avoid indirect reference to the defendant's failure to testify. No matter how delicately worded, a summation which points to the strength of the government's proof, points inevitably, albeit silently, to the weakness of the defendant's. However, unless a prosecutor's comments are of such a nature that a jury would naturally and necessarily construe them to be directed to the failure of the defendant to testify, they are not prejudiciously unfair. United States ex rel. Leak v. Follette, 418 F.2d 1266, 1269 (2d Cir. 1969), cert. denied, 397 U.S. 1050, 90 S.Ct. 1388, 25 L.Ed.2d 665 (1970).