A PR to modify `system.nim`, in the devel branch? I'm not good at using `git`.
Also, is there a preferred way to check if a closure iterator has finished iteration, by using only that two pointers? Or are they not intended to be used without the original closure variable? The way I learned from C code is `cast[ptr UncheckedArray[int]](env)[1]`, where `env` is the environment pointer, but this doesn't seem to be an API.
