discussed Cited "but see" Paul A. Zeigler v. William T. Callahan See Giglio, 450 U.S. at 154, 92 S.Ct. at 766 ; Woodcock v. Amaral, 511 F.2d at 990, n.5 ; but see Garrison v. Maggio, 540 F.2d 1271, 1274 (5th Cir.), reh. denied, 544 F.2d 518 , cert. denied, 431 U.S. 940 , 97 S.Ct. 2655 , 53 L.Ed.2d 258 (1977) (Fifth Circuit applies a more rigorous standard of materiality where evidence is useful only for impeachment).