func forperm(callback, n) {
    var idx = @^n

    loop {
        callback(idx...)

        var p = n-1
        while (idx[p-1] > idx[p]) {--p}
        p == 0 && return()

        var d = p
        idx += idx.splice(p).reverse

        while (idx[p-1] > idx[d]) {++d}
        idx.swap(p-1, d)
    }

    return()
}

forperm({|*p| say p }, 3)