Short answer: You need to use `ref History` instead of `var History`. Longer answer: A `var` argument is only valid for the duration of the call to the proc. This is why it cannot be captured by a closure (your nested proc) -- there's no guarantee that the argument would still be available upon exit. Using a `ref` allows multiple references to the same data structure and guarantees that the data won't be released (garbage collected) until all of the references go out of scope.
Hope this helps!
