[eli_gottlieb]

Actually, this issue is what spurred a major evolution in the language. I hadn't had a real region or effect system before, and now I do. Why? I needed some way to register what pointers (regions) are getting captured by an existential package (aka: a lexical closure). Since regions are now registered as part of an existential package, the regions captured by a package can be constrained to suit the region of the stack up to which a package is being returned.

In short, closures now remember what pointers they capture, and you won't be allowed to return a closure over a pointer further up the stack than safe. Other than that, closures capture by value, copying the captured variable into an environment structure.