10  GOSUB 100
20  IF f>0 THEN GOSUB 200
30  GOTO 20
99  REM ## Fill the glass
100  LET f=1
105  PRINT "Glass Filled - phew!"
110  RETURN
199  REM ## Drink some
200  LET f = f - 0.1
205  PRINT "Sip ... ahhhhhh"
210  IF f < 0 THEN GOSUB 300
220  RETURN
299  REM ## Panic !!!
300  PRINT "Oh noes - I'm out"
310  GOSUB 100
320  RETURN