Re: add-hook

[Richard Stallman <rms@gnu.org>]

    ;; Setting `local' to t ensures that the var will be modified with `set'
    ;; whereas a setting of nil implies that the hook will be modified with
    ;; `set-default', so the safe setting is t.  The only case where we
    ;; want to use the unsafe setting of nil is when we indeed want to change
    ;; the global part of the hook and there is both a local and a global part
    ;; (i.e when local == nil and when there's a t in the hook's content).

That comment may be correct, but it is still hard to understand.
Also, the variable ought to be renamed.  t does not mean the hook
is local or that anything is local.