Ooh, interesting - so it's clear that the B responded to A, not to A'. And your DAG ends up A'->B->A (where -> means responds to).

OTOH I'm not sure that cycles are even a huge problem.

They probably aren't a problem, but they might be. I can see there potentially being some graph theory optimisations on the server side you can do if you know you're dealing with a DAG.

> And your DAG ends up A'->B->A

A' would be in response to A.

Or each edit forms a new "edit" node that new replies are then connected to. Regardless, the DAG can always be preserved somehow.