2-.beta.-D-Ribofuranosyl-1,2,4-triazine-3,5(2H, 4H)-dione and (-)-(2'R,5'S)-1-[2'-Hydroxymethyl-5'-(1,3-oxathiolanyl)]cytosine 6-Azauridine and 3TC 6-Azauridine and Lamivudine AIDS-188998 AIDS188998