How to generate unique names for Tk toplevel window paths?

I need a way to generate an unused name for Tk toplevel window paths, just like #auto does it for Itcl objects.

How can I do that? Maybe Tk has a similar utility?

When I need unique widget names, I use something like this:

variable sequencecounter 0;   # Don't touch outside this code!
proc unique {{parent ""}} {
    variable sequencecounter
    while {[winfo exists [set w $parent.w$sequencecounter]]} {
        incr sequencecounter
    }
    return $w
}

That's guaranteed to return a widget name that doesn't exist. (Tk is guaranteed to run single-threaded so you know there are no nasty race conditions.) Use it like this:

set top [toplevel [unique]]
set btn [button [unique $top] -text "Hi" -command { exit }]
pack $btn