You mean like email and usenet's in-reply-to[1][2]? Sure, not many, if any, clients handle an arbitrary list of message id's there, but it's clear[3] that's wh

You mean like email and usenet's in-reply-to[1][2]? Sure, not many, if any, clients handle an arbitrary list of message id's there, but it's clear[3] that's what it's for. [1]: https://www.rfc-editor.org/rfc/rfc822#section-4.6.2 https://www.rfc-editor.org/rfc/rfc822#section-4.6.2 [2]: https://cr.yp.to/immhf/thread.html https://cr.yp.to/immhf/thread.html [3]: https://www.rfc-editor.org/rfc/rfc5322#section-3.6.4 https://www.rfc-editor.org/rfc/rfc5322#section-3.6.4