PRINT "Number to test"
    INPUT N
    IF N < 0 THEN LET N = -N
    IF N = 2 THEN GOTO 30
    IF N < 2 THEN GOTO 40
    LET F = 1
    LET J = 1
10  LET J = J + 1
    REM exploits the fact that (F mod N)*J = (F*J mod N)
    REM to do the factorial without overflowing
    LET F = F * J
    GOSUB 20
    IF J  < N - 1 THEN GOTO 10
    IF F  = N - 1 THEN PRINT "It is prime"
    IF F <> N - 1 THEN PRINT "It is not prime"
    END
20  REM modulo by repeated subtraction
    IF F < N THEN RETURN
    LET F = F - N
    GOTO 20
30  REM special case N=2
    PRINT "It is prime"
    END
40  REM zero and one are nonprimes by definition
    PRINT "It is not prime"
    END