steploop

// arg1 = loop control variable
// arg2 = iterations
// arg3 = step number
// arg4 = body
// arg5 = starting index

steploop = [
    loop $arg1 $arg2 [
        if (! (mod $$arg1 @arg3)) [
            $arg1 = (+ $$arg1 $arg5)
            @@arg4
        ]
    ]
]

// e.g.
// list = "0 1 2 3 4 5 6 7 8 9"
// steploop p (listlen $list) 2 [echo $p] 1