.S. 79, 87-88, 114 S.Ct. 2048, 129 L.Ed.2d 67 (1994) (noting that cases where the formulation of a "special federal rule" are "few and restricted"); Kamen v. Kemper Fin. Servs., Inc., 500 U.S. 90, 98, Ill S.Ct. 1711, 114 L.Ed.2d 152 (1991) (noting that a federal court "should endeavor to fill the interstices of fed