Outline or fold mode for editing Emacs Lisp code

Here's my setup:

(require 'hideshow)
(global-set-key (kbd "C-M-h") 'hs-toggle-hiding)
(global-set-key (kbd "<f2> h h") 'hs-hide-all)
(global-set-key (kbd "<f2> h j") 'hs-show-all)

outline-minor-mode does that. Look into its menu. Instead of the +-sign you get the typical ellipsis .... That should not hurt. You can do the key-bindings for yourself. (I know that you are clever enough!)