documentation bug

[Frederik Fouvry]

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In GNU Emacs 21.3.1 (i686-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2004-02-19 on cc.at.coli.uni-sb.de
configured using `configure  --prefix=/export/black --with-xpm --with-jpeg 
--with-tiff --with-gif --with-png --with-x --with-gcc --with-pop --with-sound'
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: POSIX
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: de_DE@euro
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_GB
  locale-coding-system: iso-latin-1
  default-enable-multibyte-characters: t

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

In the Elisp manual, there is the following section (in
"Conventions for Writing Minor Modes"):

     The command should accept one optional argument.  If the argument
     is `nil', it should toggle the mode (turn it on if it is off, and
     off if it is on).  Otherwise, it should turn the mode on if the
     argument is a positive integer, a symbol other than `nil' or `-',
     or a list whose CAR is such an integer or symbol; it should turn
     the mode off otherwise.

     Here is an example taken from the definition of
     `transient-mark-mode'.  It shows the use of `transient-mark-mode'
     as a variable that enables or disables the mode's behavior, and
     also shows the proper way to toggle, enable or disable the minor
     mode based on the raw prefix argument value.

          (setq transient-mark-mode
                (if (null arg) (not transient-mark-mode)
                  (> (prefix-numeric-value arg) 0)))


The given example does not behave as documented for the arguments
'(-) and '(nil):

(prefix-numeric-value '(-)) -> 1
(prefix-numeric-value '(nil)) -> 1

According to the quoted text, these arguments should switch off
the mode.

Either prefix-numeric-value() is not behaving correctly, or the
example is not complete.  In the latter case, one could write

          (setq transient-mark-mode
                (let ((larg (if (consp arg) (car arg) arg)))
                  (if (null larg) (not transient-mark-mode)
                    (> (prefix-numeric-value larg) 0))))