REF - CMD Conditional Rail

cmd = [
    arg5 = 0 // saving us the trouble of declaring a local var
    loopwhile i (listlen $arg1) [>= $arg5 0] [
        if ((at $arg1 $i)) [
            doargs (at $arg2 $i)
            arg5 = (+ $arg5 1)
        ] [
            doargs (at $arg3 $i)
            arg5 = -1
        ]
    ]
    if (= $arg5 (listlen $arg1)) [ doargs $arg4 ]
]