wl-sort-by-reply-date.el

;; Add below to ~/.wl and 'reply-date will be a candidate for wl-summary-sort
(defun wl-summary-overview-entity-compare-by-reply-date (a b)
  "Compare entity X and Y by latest date of replies."
  (flet ((string-max2
          (x y)
          (cond ((string< x y) y)
                ('t x)))
         (elmo-entity-to-number
          (x)
          (elt (cdr x) 0))
         (thread-number-get-date
          (x)
          (timezone-make-date-sortable (elmo-msgdb-overview-entity-get-date (elmo-message-entity wl-summary-buffer-elmo-folder x))))
         (thread-get-family
          (x)
          (cons x (wl-thread-entity-get-descendant (wl-thread-get-entity x))))
         (max-reply-date
          (x)
          (cond ((eq 'nil x)
                 'nil)
                ((eq 'nil (cdr x))
                 (thread-number-get-date (car x)))
                ('t
                 (string-max2 (thread-number-get-date (car x))
                              (max-reply-date (cdr x)))))))
    (string<
     (max-reply-date (thread-get-family (elmo-entity-to-number a)))
     (max-reply-date (thread-get-family (elmo-entity-to-number b))))))
(add-to-list 'wl-summary-sort-specs 'reply-date)