OK, so this may be a regression against issue174 where it seems to have
been thought of as being a bug.
It's also related to issue1241, but rollback seems to have a different
behaviour from obliterate/unrecord so I'm making a separate ticket.
Note that we *tried* to remove this behaviour in patch458, but we
restored the restriction when we discovered that it had unpleasant
So it looks like we either need to document this behaviour as a quirk,
add a flag, or find a sane way to fix it