^=z / z = 0
F/actorial / n -- nth-factorial
{
    z?+#; / n == 0 ? return 1

    =a$a$a / dup n
    - @F=a / a = F(n - 1)

    =b / b = n

    / push a * b
    b[$a]
    $b-=b
    b[*]
}

I/terative factorial / n -- nth-fctorial
{
    z?+#;   / n == 0 ? return 1

    =c$c    / c = n - 1
    $c-=c   / --c

    c (                     / while c > 0
        $c                  /   push c
        =a=ba[$b]$a-=aa[*]  /   multiply 2 toppest nums
        $c-=c               /   --c
    )
}

^++++++
^++++++ @F
^++++++ @I