- * If we've got the ID to be a reply
- * to, discard the current ID;
- * otherwise, make everything a reply
- * to that.
+ * For deep threading: make every mail
+ * a reply to the previous one, no
+ * matter what other options are set.
+ *
+ * For shallow threading:
+ *
+ * Without --cover-letter and
+ * --in-reply-to, make every mail a
+ * reply to the one before.
+ *
+ * With --in-reply-to but no
+ * --cover-letter, make every mail a
+ * reply to the <reply-to>.
+ *
+ * With --cover-letter, make every
+ * mail but the cover letter a reply
+ * to the cover letter. The cover
+ * letter is a reply to the
+ * --in-reply-to, if specified.