This middleware remembers the last URLs that the user visited on your Django-site and saves them into the `request.session`.
The fields are `currently_visiting` for the URL that is opened by the user and `last_visited` which is the URL before. Most of the time, you'll need only `last_visited`, as `currently_visiting` is just an implementation detail.
For what is this good for? Imagine, you have to implement something like JavaScripts `history.back()` or `history.forward(-1)` but without JavaScript. Things start to get difficult when using JavaScript is not possible, for whatever reason.
This snippet was created because I needed to redirect the user to the page he's been watching before clicking on the "translate" link. One other alternative would be adding the URL the user was visiting as a GET field to the "translate" link, but I hoped to find a possibility to avoid GET.
This snippet works quite well as a proof of concept, the only known wart is when the user uses tabs or multible windows on the site, things get messed up. This cannot be solved, it's a restriction imposed by the design of HTTP.