That last implementation is interesting, although it requires using the `applyc` command to invoke the closure because it has to take it by name instead of by value, which is an unfortunate limitation. The thing you can't do is make a closure that can be invoked just with `apply`. Offhand the only way I can think of to try that is to store the closure data globally under a unique name, but then there's no way to clean up that data when the closure goes away since Tcl doesn't have finalizers (you can run code when a variable goes out of scope, but you can't do it for the value itself).