** asynchronous access to the folders
** improvement of folder mode
** abbreviation of folder names (consider huge number of folders)
** independent cache mark
** History Folder (proposal by Tomotaka SUWA)
** enable to define folder icons in elmo.el
** sorted update in summary mode
** use hash table for entries in .addresses
** mitigate special treatment of NNTP around wl-folder-check-entity-async
** organize variable like wl-summary-other-frame-list so as to open
   matched summary in the new frame
** organize optional setting for pipe folders to download messages at once
   when one checks new messages
** support for rfc2192, rfc2193, rfc2221 (managing imap referral)
** putting codes in order