Issues: 1) some users can edit things other user's can't. 2) some pages (and ultimately parts of pages) can be seen only when the user has met certain criteria. 3) users are members of a cohort, which should be the real tree of the course. 4) Users may be members of multiple cohorts, and have different rights in each of them. 5) The current system can show different users different trees, and that is a mistake for several reasons, caused by me not thinking of a better mechanism.