- * We first look at 'message_file' and its link-relevant headers
- * (References and In-Reply-To) for message IDs. We also look in the
- * database for existing message that reference 'message'. In either
- * case, we will assign to the current message the first thread_id
+ * The first check is in the metadata of the database to see if we
+ * have pre-allocated a thread_id in advance for this message, (which
+ * would have happened if a message was previously added that
+ * referenced this one).
+ *
+ * Second, we look at 'message_file' and its link-relevant headers
+ * (References and In-Reply-To) for message IDs.
+ *
+ * Finally, we look in the database for existing message that
+ * reference 'message'.
+ *
+ * In all cases, we assign to the current message the first thread_id