linked-tags can represent cycles, but is that always nonsense?

[deeglaze]

It doesn't make sense to have a cycle in supplements / replaces relationships, since they are apparently causally ordered.

Since $tag-rel-type-choice is extensible by profiles, do we say that only cycles involving supplements/replaces are invalid and leave the validity of cycles to a profile that extends the relationship enum to decide? Or just disallow cycles entirely?