ourt decisions have reaffirmed the principle that the power of the federal courts to fashion principles of federal common law is limited. See, e.g., O'Melveny & Myers v. FDIC, 512 U.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 "