* elmo-shimbun.el (elmo-shimbun-check-interval): New user option.

* elmo-shimbun.el (elmo-shimbun-check-interval): New user option.
(elmo-shimbun-headers-cache): New internal variable.
(elmo-shimbun-headers-cache-header-list): New inline function.
(elmo-shimbun-headers-cache-set-header-list): Ditto.
(elmo-shimbun-headers-cache-header-hash): Ditto.
(elmo-shimbun-headers-cache-set-header-hash): Ditto.
(elmo-shimbun-headers-cache-last-check): Ditto.
(elmo-shimbun-headers-cache-set-last-check): Ditto.
(elmo-shimbun-lapse-seconds): Ditto.
(elmo-shimbun-headers-cache-check-p): Ditto.
(elmo-shimbun-get-headers): New function.
(elmo-folder-open-internal): Call it.
(elmo-quit): Define (Clear headers-cache).

* wl-summary.el (wl-summary-set-message-buffer-or-redisplay):
Check wl-message-buffer lives before set-buffer.