(defun nnmail-split-fancy-with-parent ()
"Split this message into the same group as its parent.
This function can be used as an entry in `nnmail-split-fancy', for
-example like this: (: nnmail-split-fancy)
+example like this: (: nnmail-split-fancy-with-parent)
For a message to be split, it looks for the parent message in the
References or In-Reply-To header and then looks in the message id
cache file (given by the variable `nnmail-message-id-cache-file') to