with permission from chofmann:

1. fix for bug 5796, crash on exit.  This was a bad, bad memory smudge on my part, easily fixed by doing the right ref counting in the
right places.

2. some preliminary code for M6 block transformations has leaked into this checkin.  It's safer than trying to re-code the fix above into
a fresh tree.  Unless you're making calls to do block transformations, you won't see any difference.