func permutations(callback, set, perm=[]) {
    set.is_empty && callback(perm)
    for i in ^set {
        __FUNC__(callback, [
            set[(0 ..^ i)..., (i+1 ..^ set.len)...]
        ], [perm..., set[i]])
    }
    return()
}

permutations({|p| say p }, [0,1,2])