+ "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)
+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
+see which group that message was put in. This group is returned.
+
+See the Info node `(gnus)Fancy Mail Splitting' for more details."