Module threads 

Conversations: messages tied by their Message-ID and the ids they answer or cite (In-Reply-To, References; RFC 5322 §3.6.4), never by subject alone, which would join strangers who both wrote “Hello”. A message tied to another through a third is in the same conversation.

Functions

group

The conversation of each card, as a number: equal for messages of the same conversation.

ids_of

The ids a message is tied by: its own first, then those it answers or cites.