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set the invisible property to `outline'.
(outline-minor-mode): Likewise.
Also, use `remove-from-invisibility-spec'.
(outline-flag-region): Set `outline' as the invisible property.

(outline-minor-mode): Likewise.

where an overlay extends on both sides of the region.
(outline-mark-subtree): New function.
(outline-mode-prefix-map): Put outline-mark-subtree on C-c @.

where an overlay extends on both sides of the region.

Use overlays-in, to ensure we get rid of empty overlays.

(outline-backward-same-level, outline-forward-same-level): Likewise.

(outline-flag-region): Run the hook.

backwards motion and handle beg/end of buffer properly.

(outline-mode): Set font-lock-keywords locally.

Define all the C-c commands here.
(outline-mode-menu-bar-map): New variable, holds local menu-bar keymap.
(outline-mode-map): Bind C-c and menu-bar to those maps.
(outline-minor-mode-map): Likewise use those submaps for the prefixes.

Don't call show-children if levels is 0.
No need to check eob in while condition.
Delete spurious setq of `first'.

(hide-sublevels): Rewritten faster and simpler.

(outline-on-heading-p): Use bolp, not bobp.
(outline-hide-other): Rename from hide-other.
(outline-hide-sublevels): Rename from hide-sublevels.
(outline-back-to-heading): Error if no previous heading.
(outline-next-visible-heading): Check for search failure.
(outline-hide-sublevels): Ignore text before first outline heading.
(outline-level): Count characters, not columns, so ^L is level 1.

(show-children): Use outline-back-to-heading.
(outline-next-preface): Don't retreat over \n or ^M if no more headers.