moderated Re: How on earth does Groups.io thread a topic?
The keyword being "usually." I remember the contortions that even youI don't recall at the moment.
At some point prior to the message I cited Mark had added the "one-week" rule for messages without either form of citation field (In-Reply-To or References). I think that was to work-around various email clients, especially mobile clients, which fail to use or provide those either of those header fields.