Set major mode for editing outlines with selective display.
Headings are lines which start with asterisks: one for major headings,
two for subheadings, etc. Lines not starting with asterisks are body lines.
Body text or subheadings under a heading can be made temporarily
invisible, or visible again. Invisible lines are attached to the end
of the heading, so they move with it, if the line is killed and yanked
back. A heading with text hidden under it is marked with an ellipsis (...).
Uses keymap `outline-mode-map', which is not currently defined.
M-x outline-next-visible-heading outline-next-visible-heading move by visible headings
M-x outline-previous-visible-heading outline-previous-visible-heading
M-x outline-forward-same-level outline-forward-same-level similar but skip subheadings
M-x outline-backward-same-level outline-backward-same-level
M-x outline-up-heading outline-up-heading move from subheading to heading
M-x hide-body make all text invisible (not headings).
M-x show-all make everything in buffer visible.
M-x hide-sublevels make only the first N levels of headers visible.
The remaining commands are used when point is on a heading line.
They apply to some of the body or subheadings of that heading.
M-x hide-subtree hide-subtree make body and subheadings invisible.
M-x show-subtree show-subtree make body and subheadings visible.
M-x show-children show-children make direct subheadings visible.
No effect on body, or subheadings 2 or more levels down.
With arg N, affects subheadings N levels down.
M-x hide-entry make immediately following body invisible.
M-x show-entry make it visible.
M-x hide-leaves make body under heading and under its subheadings invisible.
The subheadings remain visible.
M-x show-branches make all subheadings at all levels visible.
The variable `outline-regexp' can be changed to control what is a heading.
A line is a heading if `outline-regexp' matches something at the
beginning of the line. The longer the match, the deeper the level. Turning on outline mode calls the value of `text-mode-hook' and then of `outline-mode-hook', if they are non-nil. (fn)