0  DEF  FN E(MPTY) = SP = FIRST
 10  GOSUB 150EMPTY
 20  LET A$ = "A": GOSUB 100PUSH
 30  LET A$ = "B": GOSUB 100PUSH
 40  GOSUB 150EMPTY
 50  GOSUB 120PULL FIRST
 60  GOSUB 120PULL FIRST
 70  GOSUB 150EMPTY
 80  GOSUB 120PULL FIRST
 90  END
 100  PRINT "PUSH "A$
 110  LET S$(SP) = A$:SP = SP + 1: RETURN
 120  GOSUB 130: PRINT "POP "A$: RETURN
 130  IF  FN E(0) THEN  PRINT "POPPING FROM EMPTY QUEUE";: STOP
 140  A$ = S$(FI): FI = FI + 1 : RETURN
 150  PRINT "EMPTY? " MID$ ("YESNO",4 ^  FN E(0),3): RETURN