My recommendation is to separate the reply information from the tangle-forming aspect: instead of branch there should be the ancestors field which strictly points to some recent tip nodes, and a separate discussion-specific reply field that points to one or more tangle entries which can be quite old at the time of posting.

@cft that's what we have! root is the reply field, and branch is ancestors. We originally used the names root and branch because I wanted generic terms usable for any message types. It's been a somewhat hard fight getting app developers to include branch. No one is confused by "root". At the time I argued we should have consistent metaphores, and trees have roots and branches, and this persuaded @paul.

gatherings really should have branch, as I pointed out previously