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This chapter describes a number of other features related to the display that XEmacs presents to the user.
| 52.1 Refreshing the Screen | Clearing the screen and redrawing everything on it. | |
| 52.2 Truncation | Folding or wrapping long text lines. | |
| 52.3 The Echo Area | Where messages are displayed. | |
| 52.4 Warnings | Display of Warnings. | |
| 52.5 Invisible Text | Hiding part of the buffer text. | |
| 52.6 Selective Display | Hiding part of the buffer text (the old way). | |
| 52.7 The Overlay Arrow | Display of an arrow to indicate position. | |
| 52.8 Temporary Displays | Displays that go away automatically. | |
| 52.9 Blinking Parentheses | How XEmacs shows the matching open parenthesis. | |
| 52.10 Usual Display Conventions | The usual conventions for displaying nonprinting chars. | |
| 52.11 Display Tables | How to specify other conventions. | |
| 52.12 Beeping | Audible signal to the user. |
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The function redraw-frame redisplays the entire contents of a
given frame. See section Frames.
This function clears and redisplays frame frame.
frame defaults to the selected frame if omitted.
Normally, redisplay is preempted as normal if input arrives. However,
if optional second arg no-preempt is non-nil, redisplay
will not stop for input and is guaranteed to proceed to completion.
Even more powerful is redraw-display:
This function redraws all frames on device marked as having their
image garbled. device defaults to the selected device. If
device is t, all devices will have their frames checked.
Processing user input takes absolute priority over redisplay. If you call these functions when input is available, they do nothing immediately, but a full redisplay does happen eventually—after all the input has been processed.
Normally, suspending and resuming XEmacs also refreshes the screen. Some terminal emulators record separate contents for display-oriented programs such as XEmacs and for ordinary sequential display. If you are using such a terminal, you might want to inhibit the redisplay on resumption. See section Suspending XEmacs.
This variable controls whether XEmacs redraws the entire screen after it
has been suspended and resumed. Non-nil means yes, nil
means no.
The above functions do not actually cause the display to be updated;
rather, they clear out the internal display records that XEmacs
maintains, so that the next time the display is updated it will be
redrawn from scratch. Normally this occurs the next time that
next-event or sit-for is called; however, a display update
will not occur if there is input pending. See section Command Loop.
This function causes an immediate update of the cursor on frame, which defaults to the selected frame.
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When a line of text extends beyond the right edge of a window, the line can either be truncated or continued on the next line. When a line is truncated, this is normally shown with a ‘\’ in the rightmost column of the window on X displays, and with a ‘$’ on TTY devices. When a line is continued or “wrapped” onto the next line, this is shown with a curved arrow in the rightmost column of the window (or with a ‘\’ on TTY devices). The additional screen lines used to display a long text line are called continuation lines.
Normally, whenever line truncation is in effect for a particular window, a horizontal scrollbar is displayed in that window if the device supports scrollbars. See section Scrollbars.
Note that continuation is different from filling; continuation happens on the screen only, not in the buffer contents, and it breaks a line precisely at the right margin, not at a word boundary. See section Filling.
This buffer-local variable controls how XEmacs displays lines that
extend beyond the right edge of the window. If it is non-nil,
then XEmacs does not display continuation lines; rather each line of
text occupies exactly one screen line, and a backslash appears at the
edge of any line that extends to or beyond the edge of the window. The
default is nil.
If the variable truncate-partial-width-windows is non-nil,
then truncation is always used for side-by-side windows (within one
frame) regardless of the value of truncate-lines.
This variable is the default value for truncate-lines, for
buffers that do not have local values for it.
This variable controls display of lines that extend beyond the right
edge of the window, in side-by-side windows (see section Splitting Windows).
If it is non-nil, these lines are truncated; otherwise,
truncate-lines says what to do with them.
The backslash and curved arrow used to indicate truncated or continued lines are only defaults, and can be changed. These images are actually glyphs (see section Glyphs). XEmacs provides a great deal of flexibility in how glyphs can be controlled. (This differs from FSF Emacs, which uses display tables to control these images.)
For details, Redisplay Glyphs.
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The echo area is used for displaying messages made with the
message primitive, and for echoing keystrokes. It is not the
same as the minibuffer, despite the fact that the minibuffer appears
(when active) in the same place on the screen as the echo area. The
XEmacs Lisp Reference Manual specifies the rules for resolving conflicts
between the echo area and the minibuffer for use of that screen space
(see (xemacs)Minibuffer section ‘The Minibuffer’ in The XEmacs Lisp Reference Manual).
Such a conflicts may be avoided at all as described in Customizing Message Display.
Error messages appear in the echo area; see Errors.
You can write output in the echo area by using the Lisp printing
functions with t as the stream (see section Output Functions), or as
follows:
This function displays a one-line message in the echo area. The
argument string is similar to a C language printf control
string. See format in Conversion of Characters and Strings, for the details
on the conversion specifications. message returns the
constructed string.
In batch mode, message prints the message text on the standard
error stream, followed by a newline.
If string is nil, message clears the echo area. If
the minibuffer is active, this brings the minibuffer contents back onto
the screen immediately.
(message "Minibuffer depth is %d."
(minibuffer-depth))
-| Minibuffer depth is 0.
⇒ "Minibuffer depth is 0."
---------- Echo Area ---------- Minibuffer depth is 0. ---------- Echo Area ---------- |
In addition to only displaying a message, XEmacs allows you to label your messages, giving you fine-grained control of their display. Message label is a symbol denoting the message type. Some standard labels are:
message—default label used by the message
function;
error—default label used for reporting errors;
progress—progress indicators like
‘Converting... 45%’ (not logged by default);
prompt—prompt-like messages like ‘Isearch: foo’ (not
logged by default);
command—helper command messages like ‘Mark set’ (not
logged by default);
no-log—messages that should never be logged
Several messages may be stacked in the echo area at once. Lisp programs may access these messages, or remove them as appropriate, via the message stack.
This function displays message (a string) labeled as label, as described above.
The frame argument specifies the frame to whose minibuffer the message should be printed. This is currently unimplemented. The stdout-p argument is used internally.
(display-message 'command "Mark set") |
This function displays a message string with label label.
It is similar to message in that it accepts a printf-like
strings and any number of arguments.
;; Display a command message.
(lmessage 'command "Comment column set to %d" comment-column)
;; Display a progress message.
(lmessage 'progress "Fontifying %s... (%d)" buffer percentage)
;; Display a message that should not be logged.
(lmessage 'no-log "Done")
|
This function remove any message with the given label from the message-stack, erasing it from the echo area if it’s currently displayed there.
If a message remains at the head of the message-stack and
no-restore is nil, it will be displayed. The string which
remains in the echo area will be returned, or nil if the
message-stack is now empty. If label is nil, the entire
message-stack is cleared.
;; Show a message, wait for 2 seconds, and restore old minibuffer ;; contents. (message "A message") -| A message ⇒ "A Message" (lmessage 'my-label "Newsflash! Newsflash!") -| Newsflash! Newsflash! ⇒ "Newsflash! Newsflash!" (sit-for 2) (clear-message 'my-label) -| A message ⇒ "A message" |
Unless you need the return value or you need to specify a label,
you should just use (message nil).
This function returns the current message in the echo area, or
nil. The frame argument is currently unused.
Some of the messages displayed in the echo area are also recorded in the ‘ *Message-Log*’ buffer. Exactly which messages will be recorded can be tuned using the following variables.
This variable specifies the maximum size of the ‘ *Message-log*’ buffer.
This variable specifies the labels whose messages will not be logged. It should be a list of symbols.
This variable specifies the regular expressions matching messages that will not be logged. It should be a list of regular expressions.
Normally, packages that generate messages that might need to be ignored
should label them with progress, prompt, or no-log,
so they can be filtered by log-message-ignore-labels.
This variable determines how much time should elapse before command characters echo. Its value must be a number, which specifies the number of seconds to wait before echoing. If the user types a prefix key (such as C-x) and then delays this many seconds before continuing, the prefix key is echoed in the echo area. Any subsequent characters in the same command will be echoed as well.
If the value is zero, then command input is not echoed.
This variable controls where the cursor appears when a message is
displayed in the echo area. If it is non-nil, then the cursor
appears at the end of the message. Otherwise, the cursor appears at
point—not in the echo area at all.
The value is normally nil; Lisp programs bind it to t
for brief periods of time.
| 52.3.1 Customizing Message Display |
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Message display function specify message intended for echo area by
putting message text into " *Echo Area*" buffer. When event
loop code decides to update display after displaying the message, text
of this buffer is erased. How exactly the text will be displayed may
be affected by the following.
The function called to display echo area text. The default variable
value, redisplay-echo-area function, does that by displaying
the text in the same place on the screen as the echo area. So does
other redisplay code. User code can avoid this regardless of what
redisplay code will run afterwards by erasing text of " *Echo
Area*" buffer.
The variable value, if non-nil, is called by command loop after
erasing text of " *Echo Area*" buffer. It must clean up data
created by redisplay-echo-area-function value.
The function is called by command loop only when minibuffer was active
and message was displayed (text appeared in " *Echo Area*"
buffer). It must wait after displaying message so that user can read
it. By default, when the variable value is nil, the equivalent
of (sit-for 2) is run.
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XEmacs contains a facility for unified display of various warnings. Unlike errors, warnings are displayed in the situations when XEmacs encounters a problem that is recoverable, but which should be fixed for safe future operation.
For example, warnings are printed by the startup code when it encounters problems with X keysyms, when there is an error in ‘.emacs’, and in other problematic situations. Unlike messages, warnings are displayed in a separate buffer, and include an explanatory message that may span across several lines. Here is an example of how a warning is displayed:
(1) (initialization/error) An error has occurred while loading ~/.emacs: Symbol's value as variable is void: bogus-variable To ensure normal operation, you should investigate the cause of the error in your initialization file and remove it. Use the `-debug-init' option to XEmacs to view a complete error backtrace. |
Each warning has a class and a priority level. The class is
a symbol describing what sort of warning this is, such as
initialization, resource or key-mapping.
The warning priority level specifies how important the warning is. The
recognized warning levels, in increased order of priority, are:
debug, info, notice, warning, error,
critical, alert and emergency.
This function displays a warning message message (a string).
class should be a warning class symbol, as described above, or a
list of such symbols. level describes the warning priority level.
If unspecified, it default to warning.
(display-warning 'resource
"Bad resource specification encountered:
something like
Emacs*foo: bar
You should replace the * with a . in order to get proper behavior when
you use the specifier and/or `set-face-*' functions.")
---------- Warning buffer ----------
(1) (resource/warning) Bad resource specification encountered:
something like
Emacs*foo: bar
You should replace the * with a . in order to get proper behavior when
you use the specifier and/or `set-face-*' functions.
---------- Warning buffer ----------
|
This function displays a formatted labeled warning message. As above,
class should be the warning class symbol, or a list of such
symbols, and level should specify the warning priority level
(warning by default).
Unlike in display-warning, message may be a formatted
message, which will be, together with the rest of the arguments, passed
to format.
(lwarn 'message-log 'warning "Error caught in `remove-message-hook': %s" (error-message-string e)) |
This variable specifies the minimum level of warnings that should be generated. Warnings with level lower than defined by this variable are completely ignored, as if they never happened.
This variable specifies the minimum level of warnings that should be
displayed. Unlike log-warning-minimum-level, setting this
function does not suppress warnings entirely—they are still generated
in the ‘*Warnings*’ buffer, only they are not displayed by default.
This variable specifies a list of classes that should not be logged or displayed. If any of the class symbols associated with a warning is the same as any of the symbols listed here, the warning will be completely ignored, as it they never happened.
This variable specifies a list of classes that should not be logged or displayed. If any of the class symbols associated with a warning is the same as any of the symbols listed here, the warning will not be displayed. The warning will still logged in the *Warnings* buffer (unless also contained in ‘log-warning-suppressed-classes’), but the buffer will not be automatically popped up.
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You can make characters invisible, so that they do not appear on
the screen, with the invisible property. This can be either a
text property or a property of an overlay.
In the simplest case, any non-nil invisible property makes
a character invisible. This is the default case—if you don’t alter
the default value of buffer-invisibility-spec, this is how the
invisibility property works. This feature is much like selective
display (see section Selective Display), but more general and cleaner.
More generally, you can use the variable buffer-invisibility-spec
to control which values of the invisible property make text
invisible. This permits you to classify the text into different subsets
in advance, by giving them different invisible values, and
subsequently make various subsets visible or invisible by changing the
value of buffer-invisibility-spec.
Controlling visibility with buffer-invisibility-spec is
especially useful in a program to display the list of entries in a data
base. It permits the implementation of convenient filtering commands to
view just a part of the entries in the data base. Setting this variable
is very fast, much faster than scanning all the text in the buffer
looking for properties to change.
This variable specifies which kinds of invisible properties
actually make a character invisible.
tA character is invisible if its invisible property is
non-nil. This is the default.
Each element of the list makes certain characters invisible. Ultimately, a character is invisible if any of the elements of this list applies to it. The list can have two kinds of elements:
atomA character is invisible if its invisible property value
is atom or if it is a list with atom as a member.
(atom . t)A character is invisible if its invisible property value
is atom or if it is a list with atom as a member.
Moreover, if this character is at the end of a line and is followed
by a visible newline, it displays an ellipsis.
Ordinarily, commands that operate on text or move point do not care
whether the text is invisible. However, the user-level line motion
commands explicitly ignore invisible newlines. Since this causes a
slow-down of these commands it is turned off by default, controlled by
the variable line-move-ignore-invisible.
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Selective display is a pair of features that hide certain lines on the screen.
The first variant, explicit selective display, is designed for use in a Lisp program. The program controls which lines are hidden by altering the text. Outline mode has traditionally used this variant. It has been partially replaced by the invisible text feature (see section Invisible Text); there is a new version of Outline mode which uses that instead.
In the second variant, the choice of lines to hide is made automatically based on indentation. This variant is designed to be a user-level feature.
The way you control explicit selective display is by replacing a newline (control-j) with a carriage return (control-m). The text that was formerly a line following that newline is now invisible. Strictly speaking, it is temporarily no longer a line at all, since only newlines can separate lines; it is now part of the previous line.
Selective display does not directly affect editing commands. For
example, C-f (forward-char) moves point unhesitatingly into
invisible text. However, the replacement of newline characters with
carriage return characters affects some editing commands. For example,
next-line skips invisible lines, since it searches only for
newlines. Modes that use selective display can also define commands
that take account of the newlines, or that make parts of the text
visible or invisible.
When you write a selectively displayed buffer into a file, all the control-m’s are output as newlines. This means that when you next read in the file, it looks OK, with nothing invisible. The selective display effect is seen only within XEmacs.
This buffer-local variable enables selective display. This means that lines, or portions of lines, may be made invisible.
selective-display is t, then any portion
of a line that follows a control-m is not displayed.
selective-display is a positive integer, then
lines that start with more than that many columns of indentation are not
displayed.
When some portion of a buffer is invisible, the vertical movement
commands operate as if that portion did not exist, allowing a single
next-line command to skip any number of invisible lines.
However, character movement commands (such as forward-char) do
not skip the invisible portion, and it is possible (if tricky) to insert
or delete text in an invisible portion.
In the examples below, we show the display appearance of the
buffer foo, which changes with the value of
selective-display. The contents of the buffer do not
change.
(setq selective-display nil)
⇒ nil
---------- Buffer: foo ----------
1 on this column
2on this column
3n this column
3n this column
2on this column
1 on this column
---------- Buffer: foo ----------
(setq selective-display 2)
⇒ 2
---------- Buffer: foo ----------
1 on this column
2on this column
2on this column
1 on this column
---------- Buffer: foo ----------
|
If this buffer-local variable is non-nil, then XEmacs displays
‘…’ at the end of a line that is followed by invisible text.
This example is a continuation of the previous one.
(setq selective-display-ellipses t)
⇒ t
---------- Buffer: foo ----------
1 on this column
2on this column ...
2on this column
1 on this column
---------- Buffer: foo ----------
|
You can use a display table to substitute other text for the ellipsis (‘…’). See section Display Tables.
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The overlay arrow is useful for directing the user’s attention to a particular line in a buffer. For example, in the modes used for interface to debuggers, the overlay arrow indicates the line of code about to be executed.
This variable holds the string to display to call attention to a
particular line, or nil if the arrow feature is not in use.
Despite its name, the value of this variable can be either a string
or a glyph (see section Glyphs).
This variable holds a marker that indicates where to display the overlay arrow. It should point at the beginning of a line. The arrow text appears at the beginning of that line, overlaying any text that would otherwise appear. Since the arrow is usually short, and the line usually begins with indentation, normally nothing significant is overwritten.
The overlay string is displayed only in the buffer that this marker points into. Thus, only one buffer can have an overlay arrow at any given time.
You can do the same job by creating an extent with a
begin-glyph property. See section Properties of Extents.
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Temporary displays are used by commands to put output into a buffer and then present it to the user for perusal rather than for editing. Many of the help commands use this feature.
This function executes forms while arranging to insert any output they print into the buffer named buffer-name. The buffer is then shown in some window for viewing, displayed but not selected.
The string buffer-name specifies the temporary buffer, which
need not already exist. The argument must be a string, not a buffer.
The buffer is erased initially (with no questions asked), and it is
marked as unmodified after with-output-to-temp-buffer exits.
with-output-to-temp-buffer binds standard-output to the
temporary buffer, then it evaluates the forms in forms. Output
using the Lisp output functions within forms goes by default to
that buffer (but screen display and messages in the echo area, although
they are “output” in the general sense of the word, are not affected).
See section Output Functions.
The value of the last form in forms is returned.
---------- Buffer: foo ---------- This is the contents of foo. ---------- Buffer: foo ---------- (with-output-to-temp-buffer "foo"
(print 20)
(print standard-output))
⇒ #<buffer foo>
---------- Buffer: foo ----------
20
#<buffer foo>
---------- Buffer: foo ----------
|
If this variable is non-nil, with-output-to-temp-buffer
calls it as a function to do the job of displaying a help buffer. The
function gets one argument, which is the buffer it should display.
In Emacs versions 18 and earlier, this variable was called
temp-buffer-show-hook.
This function momentarily displays string in the current buffer at position. It has no effect on the undo list or on the buffer’s modification status.
The momentary display remains until the next input event. If the next
input event is char, momentary-string-display ignores it
and returns. Otherwise, that event remains buffered for subsequent use
as input. Thus, typing char will simply remove the string from
the display, while typing (say) C-f will remove the string from
the display and later (presumably) move point forward. The argument
char is a space by default.
The return value of momentary-string-display is not meaningful.
You can do the same job in a more general way by creating an extent with a begin-glyph property. See section Properties of Extents.
If message is non-nil, it is displayed in the echo area
while string is displayed in the buffer. If it is nil, a
default message says to type char to continue.
In this example, point is initially located at the beginning of the second line:
---------- Buffer: foo ---------- This is the contents of foo. ∗Second line. ---------- Buffer: foo ---------- (momentary-string-display "**** Important Message! ****" (point) ?\r "Type RET when done reading") ⇒ t ---------- Buffer: foo ---------- This is the contents of foo. **** Important Message! ****Second line. ---------- Buffer: foo ---------- ---------- Echo Area ---------- Type RET when done reading ---------- Echo Area ---------- |
This function works by actually changing the text in the buffer. As a result, if you later undo in this buffer, you will see the message come and go.
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This section describes the mechanism by which XEmacs shows a matching open parenthesis when the user inserts a close parenthesis.
The value of this variable should be a function (of no arguments) to
be called whenever a character with close parenthesis syntax is inserted.
The value of blink-paren-function may be nil, in which
case nothing is done.
Please note: This variable was named
blink-paren-hookin older Emacs versions, but since it is not called with the standard convention for hooks, it was renamed toblink-paren-functionin version 19.
If this variable is nil, then blink-matching-open does
nothing.
This variable specifies the maximum distance to scan for a matching parenthesis before giving up.
This variable specifies the number of seconds for the cursor to remain at the matching parenthesis. A fraction of a second often gives good results, but the default is 1, which works on all systems.
This function is the default value of blink-paren-function. It
assumes that point follows a character with close parenthesis syntax and
moves the cursor momentarily to the matching opening character. If that
character is not already on the screen, it displays the character’s
context in the echo area. To avoid long delays, this function does not
search farther than blink-matching-paren-distance characters.
Here is an example of calling this function explicitly.
(defun interactive-blink-matching-open () "Indicate momentarily the start of sexp before point." (interactive) (let ((blink-matching-paren-distance
(buffer-size))
(blink-matching-paren t))
(blink-matching-open)))
|
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The usual display conventions define how to display each character code. You can override these conventions by setting up a display table (see section Display Tables). Here are the usual display conventions:
tab-width.
ctl-arrow. If it is
non-nil, these codes map to sequences of two glyphs, where the
first glyph is the ASCII code for ‘^’. (A display table can
specify a glyph to use instead of ‘^’.) Otherwise, these codes map
just like the codes in the range 128 to 255.
The usual display conventions apply even when there is a display
table, for any character whose entry in the active display table is
nil. Thus, when you set up a display table, you need only
specify the characters for which you want unusual behavior.
These variables affect the way certain characters are displayed on the screen. Since they change the number of columns the characters occupy, they also affect the indentation functions.
This buffer-local variable controls how control characters are
displayed. If it is non-nil, they are displayed as a caret
followed by the character: ‘^A’. If it is nil, they are
displayed as a backslash followed by three octal digits: ‘\001’.
The value of this variable is the default value for ctl-arrow in
buffers that do not override it. See section The Default Value of a Buffer-Local Variable.
The value of this variable is the spacing between tab stops used for
displaying tab characters in Emacs buffers. The default is 8. Note
that this feature is completely independent from the user-settable tab
stops used by the command tab-to-tab-stop. See section Adjustable “Tab Stops”.
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You can use the display table feature to control how all 256 possible character codes display on the screen. This is useful for displaying European languages that have letters not in the ASCII character set.
The display table maps each character code into a sequence of runes, each rune being an image that takes up one character position on the screen. You can also define how to display each rune on your terminal, using the rune table.
| 52.11.1 Display Table Format | What a display table consists of. | |
| 52.11.2 Active Display Table | How XEmacs selects a display table to use. | |
| 52.11.3 Character Descriptors | Format of an individual element of a display table. |
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A display table is an array of 256 elements. (In FSF Emacs, a display
table is 262 elements. The six extra elements specify the truncation
and continuation glyphs, etc. This method is very kludgey, and in
XEmacs the variables truncation-glyph, continuation-glyph,
etc. are used. See section Truncation.)
This creates and returns a display table. The table initially has
nil in all elements.
The 256 elements correspond to character codes; the nth
element says how to display the character code n. The value
should be nil, a string, a glyph, or a vector of strings and
glyphs (see section Character Descriptors). If an element is nil,
it says to display that character according to the usual display
conventions (see section Usual Display Conventions).
If you use the display table to change the display of newline characters, the whole buffer will be displayed as one long “line.”
For example, here is how to construct a display table that mimics the
effect of setting ctl-arrow to a non-nil value:
(setq disptab (make-display-table))
(let ((i 0))
(while (< i 32)
(or (= i ?\t) (= i ?\n)
(aset disptab i (concat "^" (char-to-string (+ i 64)))))
(setq i (1+ i)))
(aset disptab 127 "^?"))
|
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The active display table is controlled by the variable
current-display-table. This is a specifier, which means
that you can specify separate values for it in individual buffers,
windows, frames, and devices, as well as a global value. It also
means that you cannot set this variable using setq; use
set-specifier instead. See section Specifiers. (FSF Emacs
uses window-display-table, buffer-display-table,
standard-display-table, etc. to control the display table.
However, specifiers are a cleaner and more powerful way of doing
the same thing. FSF Emacs also uses a different format for
the contents of a display table, using additional indirection
to a “glyph table” and such. Note that “glyph” has a different
meaning in XEmacs.)
The display table currently in use. This is a specifier.
Display tables are used to control how characters are displayed. Each
time that redisplay processes a character, it is looked up in all the
display tables that apply (obtained by calling specifier-instance
on current-display-table and any overriding display tables
specified in currently active faces). The first entry found that
matches the character determines how the character is displayed. If
there is no matching entry, the default display method is
used. (Non-control characters are displayed as themselves and control
characters are displayed according to the buffer-local variable
ctl-arrow. Control characters are further affected by
control-arrow-glyph and octal-escape-glyph.)
Each instantiator in this specifier and the display-table specifiers in faces is a display table or a list of such tables. If a list, each table will be searched in turn for an entry matching a particular character. Each display table is one of
char or generic.
Each entry in a display table should be one of
#### At some point in the near future, display tables are likely to be expanded to include other features, such as referencing characters in particular fonts and allowing the character search to continue all the way up the chain of specifier instantiators. These features are necessary to properly display Unicode characters.
Individual faces can also specify an overriding display table;
this is set using set-face-display-table. See section Faces.
If no display table can be determined for a particular window, then XEmacs uses the usual display conventions. See section Usual Display Conventions.
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Each element of the display-table vector describes how to display a particular character and is called a character descriptor. A character descriptor can be:
Display this particular string wherever the character is to be displayed.
Display this particular glyph wherever the character is to be displayed.
The vector may contain strings and/or glyphs. Display the elements of the vector one after another wherever the character is to be displayed.
nilDisplay according to the standard interpretation (see section Usual Display Conventions).
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