@mckinley@twtxt.net Actually, I think it makes no such assumptions. I believe yarnd implement the Twt Subject extension which states:

Twts in their purest form provide only the mentions mechanism to reply to certain twtxt users. This works well in small, low traffic twtxt communities. However, if there are several ongoing discussions at the same time, a single mention may not be enough for consuming twxt users to clearly identify the exact conversation > this twt is considered part of by its author. So twtxt users quickly started to provide more context in parentheses at the beginning of the twt right after any mentions – the so called subject – in the form of:

@<nick1 url1> @<nick2 url2> (re: topic) That's what I think as well.
                            ^^^^^^^^^^^
                            traditional subject

The twt subject provides a mechanism to specify references in twt replies and thus group twts into entire conversations.