(defun message-change-subject-interactively (&optional new-subj) ;gnus-subject-interactive
  "Change the Subject, bracketing with \"(was...)\" if necessary."
  (interactive "*")
  (undo-boundary)
  (save-excursion
    (save-restriction
      (message-narrow-to-headers-or-head)
      (let* ((subject (message-fetch-field "Subject"))
             (case-fold-search t))
        (if (null subject)
            (setq new-subj (read-from-minibuffer "New Subject: "))
          ;; There was already a subject
          (when (string-match "\\s-+\\'" subject) ; remove trailing spaces
            (setq subject (replace-match "" t t subject)))
          (if (and (not new-subj) (string-match "^[Rr][Ee]\\>" subject))
              ;; only ask if it wasn't our subject
              (setq new-subj
                    (read-from-minibuffer (concat "New Subject: (was " subject ") "))))
          ;; new-subj is nil if we already have an original subject (not starting with "Re:")
          ;; It is a string if we are following-up (so might want to remove "(was ...)")
          (if (and new-subj
                   (or                  ;(re-search-forward "\\s-*\\((was\\>.*\\)" subject-end t)
                    (string-match "\\s-*\\([[(,;]\\s-*was\\>.*\\)" subject)
                    (string-match "\\s-*\\<\\(was:?\\s-+[Rr][Ee]:.*\\)" subject)
                    (string-match "\\s-*[[(,;]\\s-*\\([Rr][Ee]:.*\\)" subject))
                   (or (not (string= new-subj "")) ; if replacing, force removal
                       (y-or-n-p (concat "Remove \"" (match-string 1 subject) "\" from Subject: line? "))))
              (setq new-subj (replace-match "" t t subject)
                    subject nil))
          (if (or (null new-subj) (string= "" new-subj))
              ;; No true change in subject
              (setq new-subj subject
                    subject nil)
            ;;  User changed the subject
            ;; first undo RFC-822 line-wrap
            (while (and subject (string-match "\n[ \t]" subject))
              (setq subject (replace-match " " t t subject)))
            ;; might be a followup to a forwarded message, so trim [...] too
            (while (and subject (string-match message-change-subject-ignored-prefixes subject))
              (setq subject (replace-match "" t t subject)))
            ;; Trim subject line to 75 chars (leaving room for a recipient to add "Re: ")
            ;; "Subject: "(9) + new-subj + "(was: "(6) + subject + "...)"(1 or 4)  <=75
            ;; means subject<=(59 - new-subj) or trim to (56 - new-subj)
            (if (and subject (> (length subject) (- 59 (length new-subj))))
                (let ((maxlen (- 56 (length new-subj))))
                  (if (<= maxlen 0)
                      (setq subject "...")
                    ;; keep one extra char, so we know if last char is a word-boundary
                    (setq subject (substring subject 0 (1+ maxlen)))
                    (if (string-match "\\s-*\\(\\B.\\|\\<.\\)*\\'" subject)
                        (setq subject (replace-match "..." t t subject))))))))
        (widen)
        (when new-subj
          (if (and subject (not (string= "" subject)))
              (setq new-subj (concat new-subj " (was: " subject ")")))
          (message-goto-subject)
          (message-delete-line)
          (insert "Subject: " new-subj "\n"))))))