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早期freebsd实现

How is this Emacs different from all other Emacses?  -*-Outline-*-
 (Actually, from Twenex Emacs)

* Copyright (c) 1985 Richard M. Stallman

   Permission is granted to anyone to make or distribute verbatim copies
   of this document as received, in any medium, provided that the
   copyright notice and permission notice are preserved,
   and that the distributor grants the recipient permission
   for further redistribution as permitted by this notice.

   Permission is granted to distribute modified versions
   of this document, or of portions of it,
   under the above conditions, provided also that they
   carry prominent notices stating who last changed them.

* Fundamental concepts.

** There is no concept of "typeout" in GNU Emacs.

Any time that a command wants to display some output,
it creates a buffer (usually with a name surrounded by asterisks)
and displays it in a window.

This provides some advantages:
 you can edit some more while looking at the output;
 you can copy parts of the output into other buffers.

It also has a disadvantage that you must type a command
in order to make the output disappear.
You can use C-x 1 to get rid of all windows except the
selected one.  To be more selective, you can switch to
the window you want to get rid of and then type C-x 0
(delete-window).

You also need to type a command to scroll the other
window if not all the output fits in it.  Meta-Control-v
will usually do the job.

** There is no concept of a "subsystem" in GNU Emacs.

Where Twenex Emacs would use a subsystem, GNU Emacs
instead creates a buffer and redefines commands in it.

For example, when you send mail in GNU Emacs, you use
a buffer named *mail* which is in Mail Mode.  You can
switch away from this buffer to any other buffer and
resume normal editing; then switch back and resume
composing mail.  You do not have to "exit" from
composing mail in order to do ordinary editing.

This has many advantages, but it also has a disadvantage:
Subsystems in Emacs tend to have "exit" commands that return you
to whatever you were doing before entering the subsystem.
In GNU Emacs the idea of what to return to is not well defined,
so it is not clear what an "exit" command should do.
The only way to "exit" in general is to type C-x b, C-x C-f, or
some other suitable command to switch buffers.  Some
subsystem-like major modes, such as Info and Mail mode, provide
commands to "exit" by switching to the previously selected
buffer.

** Files are always visited in their own buffers.

Beginning users of Twenex Emacs were told how to edit
using a single buffer and reading one file after another
into that buffer.  Use of a new buffer for each file was
regarded as a more advanced mode.

In GNU Emacs, the idea of using a single buffer for various
files, one by one, has been dropped, given that the address
space is expected to be large enough for many buffers.  C-x
C-f (find-file), which behaves nearly the same as in Twenex
Emacs, is in GNU Emacs the canonical way for all users to
visit files.

Various commands need to read files into Emacs in the course
of their execution.  In Twenex Emacs the user must tell them
whether to reuse buffers or create new ones, using the variable
Tags Find File.  In GNU Emacs, these commands always use
C-x C-f.

The command C-x C-v does still exist; it kills the current
buffer and reads the specified file into a new buffer.
It is equivalent to kill-buffer followed by find-file.

Since there is no reusing of buffers, there is no point in
calling the initial buffer "main".  So the initial buffer
in GNU Emacs is called "*scratch*" and is intended for typing
Lisp expressions to be evaluated.

** File name defaulting.

GNU Emacs records a separate working directory for each buffer.
Normally this is the directory on which the buffer's file
resides; for buffers not visiting any file, it is copied from
the buffer that was current when it was created.  The current buffer's
working directory can be printed with M-x pwd and set with M-x cd.

GNU Emacs shows you the default directory by inserting it in
the minibuffer when a file name is being read.  You can type
the filename you want at the end of the default as if the
default were not there, or you can edit and alter the default.

If you want file /lose/big when the default /foo/defaultdir/
has been inserted for you, you need not kill the default; simply
type at the end of it: /foo/defaultdir//lose/big.  Such a file
name is not ordinarily considered valid, but GNU Emacs
considers it equivalent to /lose/big.

Likewise, if you want file quux in your home directory, just add
~/quux to the end of the supplied text, to get
/foo/defaultdir/~/quux.  GNU Emacs sees "/~" and throws away
everything before the "~".

You can refer to environment variables also within file names.
$ followed by the environment variable name is replaced by the
variable's value.  The variable name should either be followed
by a nonalphanumeric character (which counts as part of the
file name) or be surrounded by braces {...} (which do not count
as part of the file name).  Thus, if variable USER has value "rms",
"x/$USER-foo" is expanded to "x/rms-foo", and "x${USER}foo"
is expanded to "xrmsfoo".  Note that this substitution is not
performed by the primitive file operation functions of GNU Emacs,
but rather by the interactive file name reader.  It is also
available as a separate primitive, in the function
substitute-in-file-name.

** Exit commands C-z, C-x C-c and C-x C-z.

There are two ways to exit GNU Emacs: killing and suspending.
Killing is like what Control-c does to ordinary Unix programs.
In GNU Emacs, you type C-x C-c to kill it.  (This offers to
save any modified file buffers before really killing Emacs.)
Suspending is like what Control-z does to ordinary Unix programs.
To suspend GNU Emacs, type C-x C-z, or type just C-z.
Note that C-z suspends ordinary programs instantly, but
Emacs does not suspend until it reads the C-z.

Usually it is better to suspend: once a system is smart
enough to have job control, why ever kill an editor?
You'll just have to make a new one in a minute.
This is why the convenient command C-z is provided for
suspending.

C-c is used as a prefix key for mode-specific commands and for users'
own commands.  We deliberately do not make C-c ever kill Emacs,
because it should not be so easy to do something irreversible.

** Quitting with C-g.

If you type C-g while GNU Emacs is waiting for input, it
is an ordinary command (which is defined to beep).  If you
type C-g while Lisp code is executing, it sets a flag which
causes a special signal, nearly the same as an error, to
happen atthe next safe place in Lisp execution.  This usually
has the effect of aborting the current command in a safe way.

Because at times there have been bugs causing GNU Emacs to loop
without checking the quit flag, a special feature causes
GNU Emacs to be suspended immediately if you type a second C-g
while the flag is already set.  So you can always get out
of GNU Emacs.  Normally GNU Emacs recognizes and clears the quit flag
quickly enough to prevent this from happening.

When you resume GNU Emacs after a suspension caused by multiple C-g, it
asks two questions before resuming execution:
 Checkpoint?
 Dump core?
Answer each one with `y' or `n' and a Return.
 `y' to Checkpoint? causes immediate auto-saving of all
    buffers in which auto-saving is enabled.
 `y' to Dump core? causes an illegal instruction to be executed.
    This is to enable a wizard to figure out why GNU Emacs was
    looping without checking for quits.  Execution does not continue
    after a core dump.  If you answer `n', execution continues.
With luck, GNU Emacs will ultimately check the quit flag,
and quit normally.  If not, and you type another C-g, it
is suspended again.

If GNU Emacs is not really hung, just slow, you may invoke
the double C-g feature without really meaning to.  Then just
resume and answer `n' to both questions, and you will
arrive at your former state.  Presumably the quit you
wanted will finish happening soon.

These questions are not asked if you suspend GNU Emacs with the C-z
command.  Continuing GNU Emacs after a C-z takes you straight back
into editing.

** Undoing with C-x u or C-_

You can undo many commands--up to 10,000 characters worth.
Each time you type C-x u or C-_, another command or batch of change
is undone.  Undo information is stored per buffer, and the undo
command always applies to the current buffer.  A numeric argument
serves as a repeat count.

Consecutive self-inserting characters are undone in groups of twenty.

** Different character set.

GNU Emacs does not expect anyone ever to have a keyboard in which
the Control key sets an independent bit which may accompany any
character. The only control characters that can exist are the
ASCII control characters.

There is, as a result, no "control prefix" character.

** Control-h is the Help character.

I'm amazed it took me so long to get this idea.  In Twenex Emacs, C-h
and C-b are equivalent commands, making C-h redundant.  C-h is not
only easy to type, it is mnemonic for "Help".  So in GNU Emacs the
Help character is C-h.

** Completion is done by TAB, not ESC.

ESC in the minibuffer is a Meta prefix, same as at top level.

** The string-argument reader is the minibuffer is an editor window.

In GNU Emacs, the line at the bottom of the screen is the minibuffer.
Commands that want string arguments always use this line to read them,
and you can use the ordinary Emacs editing commands to edit the
input.  You can terminate input with Return because Return is defined
as the exit-minibuffer command when in the minibuffer.  If you
are using a command that needs several arguments, terminate each
one with Return.  You cannot separate arguments with Escape
the way you would in Twenex Emacs.

The minibuffer window does not overlay other editor windows;
it is a nearly ordinary editor window which lacks a mode line
and is "turned off" when not in use.  While it IS in use, you
can switch windows to and from the minibuffer, kill text in other
windows and yank in the minibuffer, etc.

You can even issue a command that uses the minibuffer while in the
minibuffer.  This gets you temporarily into a recursive minibuffer.
However, this is allowed only if you enable it, since it could be
confusing for beginners.

When you exit the minibuffer, the cursor immediately moves back to
column zero of the minibuffer line, to show you that the exit
command has been obeyed.  The minibuffer contents remain on the screen
until the end of the command, unless some other text is displayed there.

A single Control-g exits the minibuffer.

** There are no &'s or ^R's or spaces in function names.

For example, the function which is called ^R Forward Word
in Twenex Emacs is called forward-word in GNU Emacs.

** The extension language is Lisp rather than TECO.

Libraries must be written in Lisp.  Meta-ESC reads a Lisp
expression, evaluates it, and prints the result.  Note that
Meta-ESC is "disabled" by default, so that beginning users
do not get into the minibuffer by accident in a confusing way.

Data types available include integers (which double as characters),
strings, symbols, lists, vectors, buffers, buffer pointers,
windows, and process channels.

For now, to learn about writing Lisp code for GNU Emacs, read some of
the source code, which is in directory ../lisp.  Also, all Lisp
primitives have self-documentation you can read with C-h f.

** Enabling the error handler.

GNU Emacs has a Lisp debugger/stepper/trace package, but normally
errors do not enter the debugger because that is slow, and unlikely to
be of interest to most users.  Set the variable debug-on-error to t to
cause errors to invoke the debugger.  Set debug-on-quit to cause quit
signals (caused by C-g) to invoke the debugger.

* Other changes.

** More than two windows are allowed.

C-x 2 splits the current window into two windows,
  one above the other.  Initially they both display
  the same buffer.

  C-x 2 now accepts a numeric argument to specify the number of
  lines to give to the uppermost of the two windows it makes.

C-x 0 kills the current window, making all others larger.
C-x 1 kills all windows except the current one.
C-x O switches to the next window down.
  It rotates from the bottom one to the top one.
  An argument serves as a repeat count; negative arguments
  circulate in the reverse order.

If the same buffer is displayed in several windows,
changes made in it are redisplayed in all of them.

** Side by side windows are supported.

The command C-x 5 splits the current window into
two side-by-side windows.

C-x } makes the selected window ARG columns wider at the
expense of the windows at its sides.  C-x { makes the selected
window ARG columns narrower.  An argument to C-x 5 specifies
how many columns to give to the leftmost of the two windows made.

** Horizontal scrolling of the lines in a window is implemented.

C-x < (scroll-left) scrolls all displayed lines left,
with the numeric argument (default 1) saying how far to scroll.
When the window is scrolled left, some amount of the beginning
of each nonempty line is replaced by an "$".
C-x > scrolls right.  If a window has no text hidden at the left
margin, it cannot be scrolled any farther right than that.
When nonzero leftwards scrolling is in effect in a window.
lines are automatically truncated at the window's right margin
regardless of the value of the variable truncate-lines in the
buffer being displayed.

** Return key does not use up empty lines.

In Twenex Emacs, the Return command advances over an existing
empty line in some cases.  In GNU Emacs, the Return command always
makes inserts a newline.  Twenex Emacs was designed at a time when
most display terminals did not have the ability to scroll part
of the screen, and using existing empty lines made redisplay faster.
Nowadays, terminals that cannot scroll part of the screen are rare,
so there is no need to make Return behave in a more complicated manner.

** Help m.

Typing C-h m displays documentation of the current major mode.,
telling you what special commands and features are available
and how to use them or get more information on them.

This is simply the documentation, as a function, of the
symbol which is the value of major-mode.  Each major mode
function has been given documentation intended for C-h m.

** Display-hiding features.

*** Hiding indented lines

The command C-x $ with numeric argument N causes lines indented by N
or more columns to become invisible.  All you see is " ..."  appended
to the previous line, in place of any number of consecutive invisible
lines.

*** Outline Mode.

Outline mode is designed for editing outline-structured
files, such as this one.

Headings should be lines starting with one or more asterisks.
Major headings have one asterisk, subheadings two, etc.
Lines not starting with asterisks are body text.

You can make the body under a heading, or the subheadings
under a heading, temporarily invisible, or visible again.
Invisible lines are attached to the end of the previous line
so they go with it if you kill it and yank it back.

Commands:
Meta-}   next-visible-heading      move by visible headings
Meta-{   previous-visible-heading  move by visible headings

Meta-x hide-body	make all body text invisible (not headings).
Meta-x show-all		make everything in buffer visible.

The remaining commands are used when dot is on a heading line.
They apply to some of the body or subheadings of that heading.
C-c C-h  hide-subtree	make text and subheadings invisible.
C-c C-s  show-subtree	make text and subheadings visible.
C-c C-i  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 text under heading and under its subheadings invisible.
		 The subheadings remain visible.
M-x show-branches  make all subheadings at all levels visible.

** C mode is fancy.

C mode assumes that you put the initial open-brace of
a function definition at the beginning of a line.
If you use the popular indenting style that puts this
open-brace at the end of a line containing a type declaration,