It all depends on the behavior that you want. Do you want what's saved upon exit to be the value of your global `history` variable _at the time of exit_? Or do you want it to be the value at the time you called the `persistence()` proc?
If you use the `let history = history` trick, you're making a local copy, so it will be the value when `persistence()` is called. If you want the value of the global `history` variable, then you either need to use the global variable itself or a `ref`.
