concurring.
I join Judge Beam’s opinion with respect to Parts I and III, and I concur in the portions of Part II A that address the *1056issue of opinion work product. I also concur in the judgment.
I agree entirely with Judge Beam’s well-reasoned analysis of the opinion work product doctrine issue.7 We long ago held that an “attorney’s personal recollections, notes, and memoranda” containing a witness’s oral statements are “absolutely, rather than conditionally, protected.” In re Grand Jury Proceedings (Duffy), 473 F.2d 840, 848 (8th Cir.1973). In Upjohn v. United States, 449 U.S. 383, 401, 101 S.Ct. 677, 66 L.Ed.2d 584 (1981), the United States Supreme Court declined to address the question of whether any “showing of necessity can overcome protection of work product which is based on oral statements from witnesses.” In doing so, however, it cited our In re Grand Jury Proceedings as a case holding that the protection of work product based upon oral statements from witnesses cannot be overcome by any showing of necessity. In re Grand Jury Proceedings ’ absolute protection for work product based on a witness’s oral statements remains the law in this circuit and, as a panel, we are bound by its holding. See Goff v. Burton, 7 F.3d 734, 738 (8th Cir.1993) (“One panel of the circuit may not overrule a decision issued by another panel of the court”), cert. denied, 512 U.S. 1209, 114 S.Ct. 2684, 129 L.Ed.2d 817 (1994).
Our conclusion that General Motors’ counsel’s notes are protected by the work product doctrine decides this case. We need not address the question of an “at issue” waiver exception to the attorney-client privilege. Unlike the strictly federal question work product issue, the question of the possible “at issue” waiver of the attorney-client privilege turns on an interpretation of either Michigan or Missouri state law. Bedrock principles of comity and federalism suggest that federal courts should not embark upon the task of interpreting state law unless we are required to engage in such rendition. As no such requirement exists in the instant case, I respectfully decline to join those portions of either Judge Beam’s or Judge Heaney’s opinions addressing the possible “at issue” waiver exception to the attorney client privilege. Additionally, because the appel-lees did not raise in the district court the contention on appeal that General Motors waived its opinion work product protection, the assertion of such a waiver made on appeal need not be addressed. See Carter v. Chrysler Corp., 173 F.3d 693, 704 n. 9 (8th Cir.1999).
. Judge Beam's opinion states that even if the documents in this case are deemed ordinary work product, the Bakers have not shown a substantial need for the documents because Ron Elwell can counter Ivey’s testimony. Judge Heaney’s opinion states that Elwell's testimony is not comprehensive enough to serve as a substitute for the documents. As these documents are not ordinary work product, I would not decide the issue of the comprehensiveness of Elwell’s testimony.