(-)-1-[(1R, 4S)-2,3-Dideoxy-2,3-didehydro-2-fluoro-4-thio-.beta.-D-ribofuranosyl]thymine 2,4(1H,3H)-Pyrimidinedione, 1-[(2R,5S)-3-fluoro-2,5-dihydro-5-(hydroxymethyl)-2-thienyl]-5-methyl- AIDS-164172 AIDS164172