ce undergone some erosion. See Hanna v. Plumer, 380 US. 460 (1965); Byrd v. Blue Ridge Rural Elec. Cooperative, Inc., 356 US. 525 (1958); Sunray v. Beach Aircraft Corp., 349 Fad 60 (4th Cir. 1965). 10 Guaranty Trust Co. v. York, 326 US. 99 (1945). No court in a diversity case has ever failed to apply the applicabl