342, 348 (4th Cir.1994) (opinion work product even more scrupu- lously protected" than fact work' product); National Union Fire Ins. Co. R. Murray Sheet Metal Co., 967 F.2d 980, 984 (4th Cir.1992) (opinion work product "absolutely immune" from discovery); In re Martin Marietta Corp., 856 F.2d 619, 625 (4