Indeed, it's a pity that discussion threads are trees rather than DAGs. In email and usenet, you can specify multiple entries in the "References" header, to the

Indeed, it's a pity that discussion threads are trees rather than DAGs. In email and usenet, you can specify multiple entries in the "References" header, to theoretically you can refer to multiple parent messages. (Alas, that header isn't actually used for that purpose, but just to refer to parent, grantparent, etc. to have some redundancy in case of an incomplete archive or partially-private discussions.)