I'm keeping it AND asserting at every possible location that the env
passed all the way in is the same as what the mapping produces. If in
months I haven't seen a single crash then I can evaluate which way of
passing the env around is better. (It'd be code size vs. performance,
as the passing of env is noticably faster. Code size could be fixed by
turning 'XWEnv xwe,' into a macro that goes away on some builds.)
