** 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
-** reverse sort, sorted update in summary mode
+** 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