factorial

input
+>>>, 
subtract one except if 0
[-

[ 
    copy over the factorial
    [-<+<+>>] < 
    move it back
    [->+<] < 
    multiply 
    [
        -< [-<+<+>>] < [->+<] >>
    ] <<<
    [->>+<<]
    >>>>>
    subtract one
    -
]

print
]<<<;