How does Mailman determine what thread does a given message belong to?

Hi!
I'm running a mailing list with several webmail client users who chose to receive digests. How should they properly send replies, so they get in the proper thread?
Moderator of mailman-users@python.org suggested me to write a proper Subject: thread to put reply to by combination of Subject: and In-Reply-To: headers, but I am not completely sure.
Also another user replied to the digest she received without changing the Subject: (which was also a mistake), and this also caused creating a new thread.
Can somebody please point me at the logic which Mailman uses to determine the target thread of a given message? Does it only look for In-Reply-To: header, or it uses some heuristics which can be used by digested webmail users?
Best regards, Ivan.

On 05/05/2013 06:19 AM, Krylov Ivan wrote:
There are several ways.
- Subscribe to the MIME format digest and use a mail client that understands the MIME digest format and allows you to reply to individual message attachments in the digest.
- Generate the reply by going to the message in the list's archives and replying via the mailto: link (from the author's email address)
- Make sure the Subject: of the reply matches the Subject: of the message being replied to (only works in Mailman 2.1.14 and newer).
Mailman threads first by In-Reply-To: header, second by References: header and third (only since 2.1.14) by Subject: header.
-- Mark Sapiro <mark@msapiro.net> The highway is for gamblers, San Francisco Bay Area, California better use your sense - B. Dylan

On 05/05/2013 06:19 AM, Krylov Ivan wrote:
There are several ways.
- Subscribe to the MIME format digest and use a mail client that understands the MIME digest format and allows you to reply to individual message attachments in the digest.
- Generate the reply by going to the message in the list's archives and replying via the mailto: link (from the author's email address)
- Make sure the Subject: of the reply matches the Subject: of the message being replied to (only works in Mailman 2.1.14 and newer).
Mailman threads first by In-Reply-To: header, second by References: header and third (only since 2.1.14) by Subject: header.
-- Mark Sapiro <mark@msapiro.net> The highway is for gamblers, San Francisco Bay Area, California better use your sense - B. Dylan
participants (2)
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Krylov Ivan
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Mark Sapiro