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Name | Description | Command reference | Postprocessing | Example | Compatibility | Files | Authors | See also | COLOPHON |
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groff_out(5) File Formats Manual groff_out(5)
groff_out - GNU roff device-independent output format
The fundamental operation of the troff(1) formatter is the
translation of the groff(7) input language into a series of
instructions concerned primarily with placing glyphs or geometric
objects at specific positions on a rectangular page. In the
following discussion, the term command refers to this device-
independent output language, never to the groff(7) language
intended for use by document authors. Device-independent output
commands comprise several categories: glyph output; font, color,
and text size selection; motion of the printing position; page
advancement; drawing of geometric primitives; and device control
commands, a catch-all for other operations. The last includes
directives to start and stop output, identify the intended output
device, and embed URL hyperlinks in supported output formats.
Background
As groff(1) is a wrapper program around GNU troff and
automatically calls an output driver, users seldom encounter this
format under normal circumstances. groff offers the option -z to
inhibit postprocessing such that GNU troff's output is sent to
the standard output stream just as it is when running GNU troff
directly.
The purpose of device-independent output is to facilitate the
development of postprocessors by providing a common programming
interface for all devices. It is a distinct, and much simpler,
language from that of the formatter, troff. The device-
independent output can be thought of as a “page description
language”.
In the following discussion, the term troff output describes what
is output by GNU troff, while device-independent output denotes
the language accepted by the parser that interprets this output
for the output drivers. This parser handles whitespace more
flexibly than AT&T troff's implementation, recognizes a GNU
extension to the language, and supports an obsolete construct for
compatibility; otherwise, both formats are the same. (The parser
for device-independent output can be found in the groff sources
at src/libs/libdriver/input.cpp.)
When Brian Kernighan designed AT&T troff's device-independent
output format circa 1980, he had to balance readability and
maintainability against severe constraints on file size and
transmission speed to the output device. A decade later, when
James Clark wrote groff, these constraints were no longer as
tight. GNU troff therefore emits device-independent output that
is more easily interpreted by humans than did AT&T troff.
Separation
Classical troff output had strange requirements on whitespace.
The groff output parser, however, is smart about whitespace by
making it maximally optional. The whitespace characters, i.e.,
the tab, space, and newline characters, always have a syntactical
meaning. They are never printable because spacing within the
output is always done by positioning commands.
Any sequence of space or tab characters is treated as a single
syntactical space. It separates commands and arguments, but is
only required when there would occur a clashing between the
command code and the arguments without the space. Most often,
this happens when variable length command names, arguments,
argument lists, or command clusters meet. Commands and arguments
with a known, fixed length need not be separated by syntactical
space.
A line break is a syntactical element, too. Every command
argument can be followed by whitespace, a comment, or a newline
character. Thus a syntactical line break is defined to consist
of optional syntactical space that is optionally followed by a
comment, and a newline character.
The normal commands, those for positioning and text, consist of a
single letter taking a fixed number of arguments. For historical
reasons, the parser allows stacking of such commands on the same
line, but fortunately, in groff intermediate output, every
command with at least one argument is followed by a line break,
thus providing excellent readability.
The other commands — those for drawing and device controlling —
have a more complicated structure; some recognize long command
names, and some take a variable number of arguments. So all D
and x commands were designed to request a syntactical line break
after their last argument. Only one command, ‘x X’ has an
argument that can stretch over several lines, all other commands
must have all of their arguments on the same line as the command,
i.e., the arguments may not be split by a line break.
Lines containing only spaces and/or a comment are treated as
empty and ignored.
Argument units
Some commands accept integer arguments that represent
measurements, but the scaling units of the formatter's language
are never used. Most commands assume a scaling unit of “u”
(basic units), and others use “z” (scaled points); These are
defined by the parameters specified in the device's DESC file;
see groff_font(5) and, for more on scaling units, groff(7) and
Groff: The GNU Implementation of troff, the groff Texinfo manual.
Color-related commands use dimensionless integers.
Note that single characters can have the eighth bit set, as can
the names of fonts and special characters (this is, glyphs). The
names of glyphs and fonts can be of arbitrary length. A glyph
that is to be printed will always be in the current font.
A string argument is always terminated by the next whitespace
character (space, tab, or newline); an embedded # character is
regarded as part of the argument, not as the beginning of a
comment command. An integer argument is already terminated by
the next non-digit character, which then is regarded as the first
character of the next argument or command.
Document parts
A correct intermediate output document consists of two parts, the
prologue and the body.
The task of the prologue is to set the general device parameters
using three exactly specified commands. The groff prologue is
guaranteed to consist of the following three lines (in that
order):
x T device
x res n h v
x init
with the arguments set as outlined in subsection “Device Control
Commands” below. However, the parser for the intermediate output
format is able to swallow additional whitespace and comments as
well.
The body is the main section for processing the document data.
Syntactically, it is a sequence of any commands different from
the ones used in the prologue. Processing is terminated as soon
as the first x stop command is encountered; the last line of any
groff intermediate output always contains such a command.
Semantically, the body is page oriented. A new page is started
by a p command. Positioning, writing, and drawing commands are
always done within the current page, so they cannot occur before
the first p command. Absolute positioning (by the H and
V commands) is done relative to the current page, all other
positioning is done relative to the current location within this
page.
This section describes all intermediate output commands, the
classical commands as well as the groff extensions.
Comment command
#anything⟨line-break⟩
A comment. Ignore any characters from the # character up
to the next newline. Each comment can be preceded by
arbitrary syntactical space; every command can be
terminated by a comment.
Simple commands
The commands in this subsection have a command code consisting of
a single character, taking a fixed number of arguments. Most of
them are commands for positioning and text writing. These
commands are smart about whitespace. Optionally, syntactical
space can be inserted before, after, and between the command
letter and its arguments. All of these commands are stackable,
i.e., they can be preceded by other simple commands or followed
by arbitrary other commands on the same line. A separating
syntactical space is necessary only when two integer arguments
would clash or if the preceding argument ends with a string
argument.
C id⟨white-space⟩
Typeset the glyph of the special character id. Trailing
syntactical space is necessary to allow special character
names of arbitrary length. The drawing position is not
advanced.
c c Typeset the glyph of the ordinary character c. The
drawing position is not advanced.
f n Select the font mounted at position n. n cannot be
negative.
H n Horizontally move the drawing position to n basic units
from the left edge of the page. n cannot be negative.
h n Move the drawing position right n basic units. AT&T troff
allowed negative n; GNU troff does not produce such
values, but groff's output driver library handles them.
m scheme [component ...]
Select the stroke color using the components in the color
space scheme. Each component is an integer between 0 and
65536. The quantity of components and their meanings vary
with each scheme. This command is a groff extension.
mc cyan magenta yellow
Use the CMY color scheme with components cyan,
magenta, and yellow.
md Use the default color (no components; black in most
cases).
mg gray
Use a grayscale color scheme with a component
ranging between 0 (black) and 65536 (white).
mk cyan magenta yellow black
Use the CMYK color scheme with components cyan,
magenta, yellow, and black.
mr red green blue
Use the RGB color scheme with components red,
green, and blue.
N n Typeset the glyph with index n in the current font. n is
normally a non-negative integer. The drawing position is
not advanced. The html and xhtml devices use this command
with negative n to produce unbreakable space; the absolute
value of n is taken and interpreted in basic units.
n b a Indicate a break. No action is performed; the command is
present to make the output more easily parsed. The
integers b and a describe the vertical space amounts
before and after the break, respectively. GNU troff
issues this command but groff's output driver library
ignores it. See v and V.
p n Begin a new page, setting its number to n. Each page is
independent, even from those using the same number. The
vertical drawing position is set to 0. All positioning,
writing, and drawing commands are interpreted in the
context of a page, so a p command must precede them.
s n Set type size to n scaled points (unit z in GNU troff).
AT&T troff used unscaled points (p) instead; see section
“Compatibility” below.
t xyz...⟨white-space⟩
t xyz... dummy-arg⟨white-space⟩
Typeset word xyz; that is, set a sequence of ordinary
glyphs named x, y, z, ..., terminated by a space or
newline; an optional second integer argument is ignored
(this allows the formatter to generate an even number of
arguments). Each glyph is set at the current drawing
position, and the position is then advanced horizontally
by the glyph's width. A glyph's width is read from its
metrics in the font description file, scaled to the
current type size, and rounded to a multiple of the
horizontal motion quantum. Use the C command to emplace
glyphs of special characters. The t command is a groff
extension and is output only for devices whose DESC file
contains the tcommand directive; see groff_font(5).
u n xyz...
u xyz... dummy-arg⟨white-space⟩
Typeset word xyz with track kerning. As t, but after
placing each glyph, the drawing position is further
advanced horizontally by n basic units. The u command is
a groff extension and is output only for devices whose
DESC file contains the tcommand directive; see
groff_font(5).
V n Vertically move the drawing position to n basic units from
the top edge of the page. n cannot be negative.
v n Move the drawing position down n basic units. AT&T troff
allowed negative n; GNU troff does not produce such
values, but groff's output driver library handles them.
w Indicate an inter-word space. No action is performed; the
command is present to make the output more easily parsed.
Only inter-word spaces on an output line (be they
breakable or not) are thus described; those resulting from
horizontal motion escape sequences are not. GNU troff
issues this command but groff's output driver library
ignores it. See h and H.
Graphics commands
Each graphics or drawing command in the intermediate output
starts with the letter D followed by one or two characters that
specify a subcommand; this is followed by a fixed or variable
number of integer arguments that are separated by a single space
character. A D command may not be followed by another command on
the same line (apart from a comment), so each D command is
terminated by a syntactical line break.
troff output follows the classical spacing rules (no space
between command and subcommand, all arguments are preceded by a
single space character), but the parser allows optional space
between the command letters and makes the space before the first
argument optional. As usual, each space can be any sequence of
tab and space characters.
Some graphics commands can take a variable number of arguments.
In this case, they are integers representing a size measured in
basic units u. The h arguments stand for horizontal distances
where positive means right, negative left. The v arguments stand
for vertical distances where positive means down, negative up.
All these distances are offsets relative to the current location.
Unless indicated otherwise, each graphics command directly
corresponds to a similar groff \D escape sequence; see groff(7).
Unknown D commands are assumed to be device-specific. Its
arguments are parsed as strings; the whole information is then
sent to the postprocessor.
In the following command reference, the syntax element ⟨line-
break⟩ means a syntactical line break as defined in subsection
“Separation” above.
D~ h1 v1 h2 v2 ... hn vn⟨line-break⟩
Draw B-spline from current position to offset (h1, v1),
then to offset (h2, v2) if given, etc., up to (hn, vn).
This command takes a variable number of argument pairs;
the current position is moved to the terminal point of the
drawn curve.
Da h1 v1 h2 v2⟨line-break⟩
Draw arc from current position to (h1, v1)+(h2, v2) with
center at (h1, v1); then move the current position to the
final point of the arc.
DC d⟨line-break⟩
DC d dummy-arg⟨line-break⟩
Draw a solid circle using the current fill color with
diameter d (integer in basic units u) with leftmost point
at the current position; then move the current position to
the rightmost point of the circle. An optional second
integer argument is ignored (this allows the formatter to
generate an even number of arguments). This command is a
groff extension.
Dc d⟨line-break⟩
Draw circle line with diameter d (integer in basic
units u) with leftmost point at the current position; then
move the current position to the rightmost point of the
circle.
DE h v⟨line-break⟩
Draw a solid ellipse in the current fill color with a
horizontal diameter of h and a vertical diameter of v
(both integers in basic units u) with the leftmost point
at the current position; then move to the rightmost point
of the ellipse. This command is a groff extension.
De h v⟨line-break⟩
Draw an outlined ellipse with a horizontal diameter of h
and a vertical diameter of v (both integers in basic
units u) with the leftmost point at current position; then
move to the rightmost point of the ellipse.
DF color-scheme [component ...]⟨line-break⟩
Set fill color for solid drawing objects using different
color schemes; the analogous command for setting the color
of text, line graphics, and the outline of graphic objects
is m. The color components are specified as integer
arguments between 0 and 65536. The number of color
components and their meaning vary for the different color
schemes. These commands are generated by the groff escape
sequences \D'F ...' and \M (with no other corresponding
graphics commands). This command is a groff extension.
DFc cyan magenta yellow⟨line-break⟩
Set fill color for solid drawing objects using the
CMY color scheme, having the 3 color components
cyan, magenta, and yellow.
DFd ⟨line-break⟩
Set fill color for solid drawing objects to the
default fill color value (black in most cases). No
component arguments.
DFg gray⟨line-break⟩
Set fill color for solid drawing objects to the
shade of gray given by the argument, an integer
between 0 (black) and 65536 (white).
DFk cyan magenta yellow black⟨line-break⟩
Set fill color for solid drawing objects using the
CMYK color scheme, having the 4 color components
cyan, magenta, yellow, and black.
DFr red green blue⟨line-break⟩
Set fill color for solid drawing objects using the
RGB color scheme, having the 3 color components
red, green, and blue.
Df n⟨line-break⟩
The argument n must be an integer in the range -32767 to
32767.
0≤n≤1000
Set the color for filling solid drawing objects to
a shade of gray, where 0 corresponds to solid
white, 1000 (the default) to solid black, and
values in between to intermediate shades of gray;
this is obsoleted by command DFg.
n<0 or n>1000
Set the filling color to the color that is
currently being used for the text and the outline,
see command m. For example, the command sequence
mg 0 0 65536
Df -1
sets all colors to blue.
This command is a groff extension.
Dl h v⟨line-break⟩
Draw line from current position to offset (h, v) (integers
in basic units u); then set current position to the end of
the drawn line.
Dp h1 v1 h2 v2 ... hn vn⟨line-break⟩
Draw a polygon line from current position to offset
(h1, v1), from there to offset (h2, v2), etc., up to
offset (hn, vn), and from there back to the starting
position. For historical reasons, the position is changed
by adding the sum of all arguments with odd index to the
current horizontal position and the even ones to the
vertical position. Although this doesn't make sense it is
kept for compatibility. This command is a groff
extension.
DP h1 v1 h2 v2 ... hn vn⟨line-break⟩
The same macro as the corresponding Dp command with the
same arguments, but draws a solid polygon in the current
fill color rather than an outlined polygon. The position
is changed in the same way as with Dp. This command is a
groff extension.
Dt n⟨line-break⟩
Set the current line thickness to n (an integer in basic
units u) if n>0; if n=0 select the smallest available line
thickness; otherwise, the line thickness is made
proportional to the type size, which is the default. For
historical reasons, the horizontal position is changed by
adding the argument to the current horizontal position,
while the vertical position is not changed. Although this
doesn't make sense, it is kept for compatibility. This
command is a groff extension.
Device control commands
Each device control command starts with the letter x followed by
a space character (optional or arbitrary space/tab in groff) and
a subcommand letter or word; each argument (if any) must be
preceded by a syntactical space. All x commands are terminated
by a syntactical line break; no device control command can be
followed by another command on the same line (except a comment).
The subcommand is basically a single letter, but to increase
readability, it can be written as a word, i.e., an arbitrary
sequence of characters terminated by the next tab, space, or
newline character. All characters of the subcommand word but the
first are simply ignored. For example, troff outputs the
initialization command x i as x init and the resolution command
x r as x res. But writings like x i_like_groff and
x roff_is_groff are accepted as well to mean the same commands.
In the following, the syntax element ⟨line-break⟩ means a
syntactical line break as defined in subsection “Separation”
above.
xF name⟨line-break⟩
(Filename control command)
Use name as the intended name for the current file in
error reports. This is useful for remembering the
original file name when groff uses an internal piping
mechanism. The input file is not changed by this command.
This command is a groff extension.
xf n s⟨line-break⟩
(font control command)
Mount font position n (a non-negative integer) with font
named s (a text word); see groff_font(5).
xH n⟨line-break⟩
(Height control command)
Set character height to n (a positive integer in scaled
points z). Classical troff used the unit points (p)
instead; see section “Compatibility” below.
xi ⟨line-break⟩
(init control command)
Initialize device. This is the third command of the
prologue.
xp ⟨line-break⟩
(pause control command)
Parsed but ignored. The classical documentation reads
pause device, can be restarted.
xr n h v⟨line-break⟩
(resolution control command)
Resolution is n, while h is the minimal horizontal motion,
and v the minimal vertical motion possible with this
device; all arguments are positive integers in basic
units u per inch. This is the second command of the
prologue.
xS n⟨line-break⟩
(Slant control command)
Set slant to n degrees (an integer in basic units u).
xs ⟨line-break⟩
(stop control command)
Terminates the processing of the current file; issued as
the last command of any intermediate troff output.
xt ⟨line-break⟩
(trailer control command)
Generate trailer information, if any. In groff, this is
currently ignored.
xT xxx⟨line-break⟩
(Typesetter control command)
Set the name of the output driver to xxx, a sequence of
non-whitespace characters terminated by whitespace. The
possible names correspond to those of groff's -T option.
This is the first command of the prologue.
xu n⟨line-break⟩
(underline control command)
Configure underlining of spaces. If n is 1, start
underlining of spaces; if n is 0, stop underlining of
spaces. This is needed for the cu request in nroff mode
and is ignored otherwise. This command is a groff
extension.
xX anything⟨line-break⟩
(X-escape control command)
Send string anything uninterpreted to the device. If the
line following this command starts with a + character this
line is interpreted as a continuation line in the
following sense. The + is ignored, but a newline
character is sent instead to the device, the rest of the
line is sent uninterpreted. The same applies to all
following lines until the first character of a line is not
a + character. This command is generated by the groff
escape sequence \X. The line-continuing feature is a
groff extension.
Obsolete command
In classical troff output, emitting a single glyph was mostly
done by a very strange command that combined a horizontal move
and the printing of a glyph. It didn't have a command code, but
is represented by a 3-character argument consisting of exactly
2 digits and a character.
ddc Move right dd (exactly two decimal digits) basic units u,
then print glyph with single-letter name c.
In groff, arbitrary syntactical space around and within
this command is allowed to be added. Only when a
preceding command on the same line ends with an argument
of variable length a separating space is obligatory. In
classical troff, large clusters of these and other
commands were used, mostly without spaces; this made such
output almost unreadable.
For modern high-resolution devices, this command does not make
sense because the width of the glyphs can become much larger than
two decimal digits. In groff, it is used only for output to the
X75, X75-12, X100, and X100-12 devices. For others, the commands
t and u provide greater functionality and superior
troubleshooting capacity.
The roff postprocessors are programs that have the task to
translate the intermediate output into actions that are sent to a
device. A device can be some piece of hardware such as a
printer, or a software file format suitable for graphical or text
processing. The groff system provides powerful means that make
the programming of such postprocessors an easy task.
There is a library function that parses the intermediate output
and sends the information obtained to the device via methods of a
class with a common interface for each device. So a groff
postprocessor must only redefine the methods of this class. For
details, see the reference in section “Files” below.
This section presents the intermediate output generated from the
same input for three different devices. The input is the
sentence hell world fed into groff on the command line.
• High-resolution device ps
shell> echo "hell world" | groff -Z -T ps
x T ps
x res 72000 1 1
x init
p1
x font 5 TR
f5
s10000
V12000
H72000
thell
wh2500
tw
H96620
torld
n12000 0
x trailer
V792000
x stop
This output can be fed into the postprocessor grops(1) to get its
representation as a PostScript file, or gropdf(1) to output
directly to PDF.
• Low-resolution device latin1
This is similar to the high-resolution device except that the
positioning is done at a minor scale. Some comments (lines
starting with #) were added for clarification; they were not
generated by the formatter.
shell> "hell world" | groff -Z -T latin1
# prologue
x T latin1
x res 240 24 40
x init
# begin a new page
p1
# font setup
x font 1 R
f1
s10
# initial positioning on the page
V40
H0
# write text 'hell'
thell
# inform about a space, and do it by a horizontal jump
wh24
# write text 'world'
tworld
# announce line break, but do nothing because ...
n40 0
# ... the end of the document has been reached
x trailer
V2640
x stop
This output can be fed into the postprocessor grotty(1) to get a
formatted text document.
• Classical style output
As a computer monitor has a very low resolution compared to
modern printers the intermediate output for the X devices can
use the jump-and-write command with its 2-digit displacements.
shell> "hell world" | groff -Z -T X100
x T X100
x res 100 1 1
x init
p1
x font 5 TR
f5
s10
V16
H100
# write text with old-style jump-and-write command
ch07e07l03lw06w11o07r05l03dh7
n16 0
x trailer
V1100
x stop
This output can be fed into the postprocessor xditview(1x) or
gxditview(1) for displaying in X.
Due to the obsolete jump-and-write command, the text clusters in
the classical output are almost unreadable.
The intermediate output language of the classical troff was first
documented in [CSTR #97]. The groff intermediate output format
is compatible with this specification except for the following
features.
• The classical quasi device independence is not yet
implemented.
• The old hardware was very different from what we use today.
So the groff devices are also fundamentally different from the
ones in classical troff. For example, the classical
PostScript device was called post and had a resolution of 720
units per inch, while groff's ps device has a resolution of
72000 units per inch. Maybe, by implementing some rescaling
mechanism similar to the classical quasi device independence,
these could be integrated into modern groff.
• The B-spline command D~ is correctly handled by the
intermediate output parser, but the drawing routines aren't
implemented in some of the postprocessor programs.
• The argument of the commands s and x H has the implicit unit
scaled point z in groff, while classical troff had point (p).
This isn't an incompatibility, but a compatible extension, for
both units coincide for all devices without a sizescale
parameter, including all classical and the groff text devices.
The few groff devices with a sizescale parameter either did
not exist, had a different name, or seem to have had a
different resolution. So conflicts with classical devices are
very unlikely.
• The position changing after the commands Dp, DP, and Dt is
illogical, but as old versions of groff used this feature it
is kept for compatibility reasons.
The differences between groff and classical troff are documented
in groff_diff(7).
/usr/local/share/groff/1.23.0/font/devname/DESC
describes the output device name.
James Clark wrote an early version of this document, which
described only the differences between AT&T device-independent
troff's output format and that of GNU troff. It has since been
expanded and revised by Bernd Warken ⟨groff-bernd.warken-72@web
.de⟩ and G. Branden Robinson ⟨[email protected]⟩.
Groff: The GNU Implementation of troff, by Trent A. Fisher and
Werner Lemberg, is the primary groff manual. You can browse it
interactively with “info groff”.
“Troff User's Manual” by Joseph F. Ossanna, 1976 (revised by
Brian W. Kernighan, 1992), AT&T Bell Laboratories Computing
Science Technical Report No. 54, widely called simply “CSTR #54”,
documents the language, device and font description file formats,
and device-independent output format referred to collectively in
groff documentation as “AT&T troff”.
“A Typesetter-independent TROFF” by Brian W. Kernighan, 1982,
AT&T Bell Laboratories Computing Science Technical Report No. 97,
(CSTR #97), provides additional insights into the device and font
description file formats and device-independent output format.
groff(1)
documents the -Z option and contains pointers to further
groff documentation.
groff(7)
describes the groff language, including its escape
sequences and system of units.
groff_font(5)
details the device scaling parameters of device DESC
files.
troff(1)
generates the language documented here.
roff(7)
presents historical aspects and the general structure of
roff systems.
groff_diff(7)
enumerates differences between the output of AT&T troff
and that of GNU troff.
gxditview(1)
is a viewer for device-independent troff output.
Roff.js
⟨https://github.com/Alhadis/Roff.js/⟩ is a viewer for
device-independent troff output written in JavaScript.
grodvi(1), grohtml(1), grolbp(1), grolj4(1), gropdf(1), grops(1),
and grotty(1) are groff postprocessors.
This page is part of the groff (GNU troff) project. Information
about the project can be found at
⟨http://www.gnu.org/software/groff/⟩. If you have a bug report
for this manual page, see ⟨http://www.gnu.org/software/groff/⟩.
This page was obtained from the project's upstream Git repository
⟨https://git.savannah.gnu.org/git/groff.git⟩ on 2024-06-14. (At
that time, the date of the most recent commit that was found in
the repository was 2024-06-10.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
groff 1.23.0.1273-9d53-dirty 6 June 2024 groff_out(5)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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