From: testfrom@example.com 
To: testto@example.com
Subject: Re: child 2.0.0
Message-Id: <child2.0.0@msg.id>
References: <root2@msg.id>  <nonexistant@msg.id>
In-Reply-To: <nonexistant@msg.id>
Date: Tue, 21 Jun 2011 15:02 +0000

abc

note, there's no message for 'nonexistant@msg.id', so this msg should
be promoted to level 2.0