From: Alice <alice@example.org>
To: Daniel <daniel@example.org>
Subject: child message
Message-ID: <B01-child@example.org>
In-Reply-To: <B00-root@example.org>
References:  <B00--root@example.org>
Date: Fri, 17 Jun 2016 22:14:41 -0400

This is a normal-ish reply, and has both a references header and an
in-reply-to header.