Thanks.

On reflection, I think the docs could do with expanding, particularly on the subject of what non-posix mode actually does and what it's useful for (it's becoming increaingly clear to me that it's nothing like a DOS shell, having used it a bit). The list of rules is all very well but doesn't mean much unless you know why it's like that. There is a cryptic comment about posix=False implying "compatibility mode"  but this concept is not explained.

The fact is that in today's world (now that the Mac is UNIX) "posix=False" more or less means "windows=True". At least unless you document very carefully what it's intended to mean. As you've probably noticed, it confused me thoroughly because I couldn't see what else it could be for, and I still haven't really grasped it.