A SIX-MONTH SAFETY AND ANTIVIRAL STUDY IN HIV-1 SEROPOSITIVE, AZT-EXPERIENCED PATIENTS WITH CD4 COUNTS LESS THAN OR EQUAL TO 50 CELLS/MM3 TO EVALUATE MK-639 ALONE VERSUS ZIDOVUDINE (AZT) AND 3TC VERSUS THE COMBINATION OF MK-639 WITH AZT/3TC: