This project is called DiscDAG, and the description says you can reply to multiple threads all at once. Too bad the example image only shows branching and not merging. It hides the key differentiator of the system.<p>If you only happen to allow branching but not merging, then the result can be represented in a tree. Indeed, this is how Hacker News, Reddit, Slashdot, and mailing lists visually present their threaded discussions.<p>Also note that email has a header named "References" which can point to multiple email message IDs, so there is some existing infrastructure that can express a DAG. Too bad no email clients use it this way; every client I've seen will visually attach each email message to zero or one parent.