This is actually the point (otherwise `Cell` would be useless). However the problem is more drastic, I still have a hard time defining how those "leaks" should be checked, and there are horrid situations like injecting a closure with a reference on the object that will pattern-match an internal enum one hand and assign it on the other...

In the end, it seems that for arbitrary types to be contained within a `SafelyMutable` little can be done safely except assignment. I am thinking of splitting the section between POD (for which it's always safe) and moving the idea of generalizing it to non-POD to the *Opened Doors* section.