79, 87-88, 114 S.Ct. 20487 129 L.Ed.2d 67 (1994) (noting that cases where the formula- tion of a "special federal rule" are "few and restricted"); Kamen I Kemper Fin. Servs., Inc., 500 U.S. 90, sir, 111 S.Ct. 1711, 114 L.Ed.2d 152 (1991) (noting that a federal court "should endeavor to fdl the interstices of fe