🚀 go-pugleaf

[version 19.25.94 sgi-mips-irix5.2]

I've found what I believe is the problem
and it isn't specific to mh-e:

   (set-mark nil) calls deactivate-mark, however deactivate-mark does
   (setq mark-active nil) ONLY if transient-mark-mode is non-nil.

So if mark-active happens to be t, and transient-mark-mode is nil
and one does (set-mark nil), this gets us into the forbidden state.
The comment that this only makes a difference in transient mark mode
should probably go away.  A suggested patch is below.  

Documentation for mark-marker should warn one to either set
mark-active to nil or call deactivate-mark if one is directly setting
mark-marker to nil.  Fortunately, in the only places other from
simple.el where this is happening outside of set-mark (allout.el and
sendmail.el), people seem to know about this.

If you want to see this manifest itself in mh-e, do, 
(on a fresh emacs -q making sure transient-mark-mode is off)

  M-x mh-rmail           ;
  C-u M-r last:5         ; (if you need to get it to show some headers...)
  .                      ; show a message
  C-x o C-space C-x o    ; go to the show buffer and set a mark
  p                      ; go to another message
  r sender               ; start a draft
  C-c C-y                ; try to yank


*** /usr/local/src/emacs-19/lisp/simple.el	Sun Aug  7 11:04:38 1994
--- -	Fri Aug 26 13:13:00 1994
***************
*** 1306,1315 ****
  (defsubst deactivate-mark ()
    "Deactivate the mark by setting `mark-active' to nil.
! \(That makes a difference only in Transient Mark mode.)
! Also runs the hook `deactivate-mark-hook'."
    (if transient-mark-mode
!       (progn
!         (setq mark-active nil)
!         (run-hooks 'deactivate-mark-hook))))
  
  (defun set-mark (pos)
--- 1306,1313 ----
  (defsubst deactivate-mark ()
    "Deactivate the mark by setting `mark-active' to nil.
! Also run the hook `deactivate-mark-hook' if in Transient Mark mode."
!   (setq mark-active nil)
    (if transient-mark-mode
!       (run-hooks 'deactivate-mark-hook)))
  
  (defun set-mark (pos)