+ * `oldsaved' list to see if it is old.
+ *
+ * Each time a message-id is seen, we mark it with MARK_SEEN.
+ *
+ * Each time a message is deleted, we mark its id UID_DELETED in the
+ * `newsaved' member. When we want to assert that an expunge has been
+ * done on the server, we call expunge_uid() to register that all
+ * deleted messages are gone by marking them UID_EXPUNGED.
+ *
+ * At the end of the query, the `newsaved' member becomes the
+ * `oldsaved' list. The old `oldsaved' list is freed.
+ *
+ * At the end of the fetchmail run, seen and non-EXPUNGED members of all
+ * current `oldsaved' lists are flushed out to the .fetchids file to
+ * be picked up by the next run. If there are no un-expunged
+ * messages, the file is deleted.
+ *
+ * One disadvantage of UIDL is that all the UIDs have to be downloaded
+ * before a search for new messages can be done. Typically, new messages
+ * are appended to mailboxes. Hence, downloading all UIDs just to download
+ * a few new mails is a waste of bandwidth. If new messages are always at
+ * the end of the mailbox, fast UIDL will decrease the time required to
+ * download new mails.
+ *
+ * During fast UIDL, the UIDs of all messages are not downloaded! The first
+ * unseen message is searched for by using a binary search on UIDs. UIDs
+ * after the first unseen message are downloaded as and when needed.
+ *
+ * The advantages of fast UIDL are (this is noticeable only when the
+ * mailbox has too many mails):