bug#24706: 26.0.50; Minor mode functions should do strict argument type checking

[Drew Adams]

> >> (define-minor-mode foo-mode nil)
> >> its docstring will be
> >>
> >> "Toggle Foo mode on or off.
> >> With a prefix argument ARG, enable Foo mode if ARG is
> >> positive, and disable it otherwise.  If called from Lisp, enable
> >> the mode if ARG is omitted or nil, and toggle it if ARG is ‘toggle’."
> >>
> >> This appears to indicate that (foo-mode 'banana)
> >> should disable foo-mode, but it enables it.
> >
> > No, it does not suggest that.  But to be clearer, it should
> > probably explicitly address the non-nil and non-`toggle' case,
> > like so:
> >
> >   If called from Lisp, enable the mode if ARG is omitted or
> >   nil, toggle it if ARG is ‘toggle’, and disable it if ARG is
> >   any other non-nil value.
> 
> If the Lisp description is meant to be exhaustive by itself, we should
> cover the numeric case too:
> 
>    If called from Lisp, enable the mode if ARG is omitted, nil, or a
>    positive number; toggle it if ARG is `toggle'; and disable it
>    otherwise.

Yes, and anyway what I wrote was wrong.

The doc string of `define-minor-mode' has it right, as does your
suggestion:

  When called from Lisp, the mode command toggles the mode if the
  argument is `toggle', disables the mode if the argument is a
  non-positive integer, and enables the mode otherwise (including
  if the argument is omitted or nil or a positive integer).

I'd suggest the same order as in the `define-minor-mode' doc:

  When called from Lisp:
  * Toggle the mode if ARG is `toggle'.
  * Disable it if ARG is a non-positive integer.
  * Enable it if ARG is anything else.