What we're understanding here is that you want to say "Communication B is 'partOf' Communication A", however you want to be able to differentiate whether that linkage is asserting "A is the overall communication thread B is part of" as opposed to "A is the first message in the thread". We're not thrilled with that notion of tying together Communication into threads and certainly don't think this belongs as a new core structure.  The second scenario "B points to A as first message in the thread" would not be a correct use of 'partOf'".  Instead, you'd need to trace the "inResponseTo" chain, or perhaps have an extension that points to the first of the thread.  There's no 'composition' relationship there. If our understanding is incorrect, please resubmit with more clarity.