bug#10773: set-variable can't change values of user options

[Glenn Morris]

Juri Linkov wrote:

> Why users are allowed to change the value of `outline-mode-hook',
> but not `outline-minor-mode-hook'?

The former is defined with (defvar ... "*...").
The latter is not defined anywhere, but as define-derived-mode says
about hooks and has been covered several times:

  No problems result if this variable is not bound.
  `add-hook' automatically binds it.  (This is true for all hook variables.)

I would says it's not very useful to pass a hook to set-variable anyway,
since you need to type a lisp expression, and probably should use
an explicit add-hook statement.