Provided by: groff_1.22.3-7_amd64 bug

NAME

       groff_diff - differences between GNU troff and classical troff

DESCRIPTION

       This  manual  page  describes  the  language  differences between groff, the GNU roff text
       processing system, and the classical roff formatter of the freely available Unix 7 of  the
       1970s,  documented in the Troff User's Manual by Ossanna and Kernighan.  This includes the
       roff language as well as the intermediate output format (troff output).

       The section SEE ALSO gives pointers to both  the  classical  roff  and  the  modern  groff
       documentation.

GROFF LANGUAGE

       In  this  section, all additional features of groff compared to the classical Unix 7 troff
       are described in detail.

   Long names
       The names  of  number  registers,  fonts,  strings/macros/diversions,  special  characters
       (glyphs),  and  colors  can  be  of  any length.  In escape sequences, additionally to the
       classical ‘(xx’ construction for a two-character glyph name, you can  use  ‘[xxx]’  for  a
       name of arbitrary length.

       \[xxx] Print the special character (glyph) called xxx.

       \[comp1 comp2 ...]
              Print  composite  glyph  consisting  of multiple components.  Example: ‘\[A ho]’ is
              capital letter A with ogonek which finally maps to glyph  name  ‘u0041_0328’.   See
              the  groff  info  file  for  details  how  a  glyph  name  for a composite glyph is
              constructed, and groff_char(7)  for  a  list  of  glyph  name  components  used  in
              composite glyph names.

       \f[xxx]
              Set  font  xxx.   Additionally,  \f[]  is  a new syntax form equal to \fP, i.e., to
              return to the previous font.

       \*[xxx arg1 arg2 ...]
              Interpolate string xxx, taking arg1, arg2, ..., as arguments.

       \n[xxx]
              Interpolate number register xxx.

   Fractional point sizes
       A scaled point is equal to 1/sizescale points, where sizescale is specified  in  the  DESC
       file  (1 by default).  There is a new scale indicator z that has the effect of multiplying
       by sizescale.  Requests and escape sequences in troff interpret arguments that represent a
       point  size as being in units of scaled points, but they evaluate each such argument using
       a default scale indicator of z.  Arguments treated in this way are the argument to the  ps
       request,  the third argument to the cs request, the second and fourth arguments to the tkf
       request, the argument to the \H escape sequence, and  those  variants  of  the  \s  escape
       sequence that take a numeric expression as their argument.

       For example, suppose sizescale is 1000; then a scaled point is equivalent to a millipoint;
       the call .ps 10.25 is equivalent to .ps 10.25z and so sets the point size to 10250  scaled
       points, which is equal to 10.25 points.

       The number register \n[.s] returns the point size in points as decimal fraction.  There is
       also a new number register \n[.ps] that returns the point size in scaled points.

       It would make no sense to use the z scale indicator in a numeric expression whose  default
       scale indicator was neither u nor z, and so troff disallows this.  Similarly it would make
       no sense to use a scaling indicator other than z  or  u  in  a  numeric  expression  whose
       default scale indicator was z, and so troff disallows this as well.

       There  is  also  new scale indicator s which multiplies by the number of units in a scaled
       point.  So, for example, \n[.ps]s is equal to 1m.  Be  sure  not  to  confuse  the  s  and
       z scale indicators.

   Numeric expressions
       Spaces are permitted in a number expression within parentheses.

       M  indicates  a  scale  of 100ths of an em.  f indicates a scale of 65536 units, providing
       fractions for color definitions with the defcolor request.  For example, 0.5f = 32768u.

       e1>?e2 The maximum of e1 and e2.

       e1<?e2 The minimum of e1 and e2.

       (c;e)  Evaluate e using c as the default scaling  indicator.   If  c  is  missing,  ignore
              scaling indicators in the evaluation of e.

   New escape sequences
       \A'anything'
              This  expands  to  1 or 0, depending on whether anything is or is not acceptable as
              the name of a string, macro, diversion,  number  register,  environment,  font,  or
              color.   It  returns 0 if anything is empty.  This is useful if you want to look up
              user input in some sort of associative table.

       \B'anything'
              This expands to 1 or 0, depending on whether anything is or is not a valid  numeric
              expression.  It returns 0 if anything is empty.

       \C'xxx'
              Typeset glyph named xxx.  Normally it is more convenient to use \[xxx].  But \C has
              the advantage that it is compatible with recent versions of UNIX and  is  available
              in compatibility mode.

       \E     This  is equivalent to an escape character, but it is not interpreted in copy mode.
              For example, strings to start and end superscripting could be defined like this

                     .ds { \v'-.3m'\s'\En[.s]*6u/10u'
                     .ds } \s0\v'.3m'

              The use of \E ensures that these definitions work even if \*{ gets  interpreted  in
              copy mode (for example, by being used in a macro argument).

       \Ff
       \F(fm
       \F[fam]
              Change  font  family.   This is the same as the fam request.  \F[] switches back to
              the previous font family (note that \FP won't work;  it  selects  font  family  ‘P’
              instead).

       \mx
       \m(xx
       \m[xxx]
              Set drawing color.  \m[] switches back to the previous color.

       \Mx
       \M(xx
       \M[xxx]
              Set  background  color  for  filled  objects drawn with the \D'...' commands.  \M[]
              switches back to the previous color.

       \N'n'  Typeset the glyph with index n in the current font.  n can be  any  integer.   Most
              devices  only have glyphs with indices between 0 and 255.  If the current font does
              not contain a glyph with that code, special fonts are not searched.  The \N  escape
              sequence can be conveniently used in conjunction with the char request, for example

                     .char \[phone] \f(ZD\N'37'

              The  index of each glyph is given in the fourth column in the font description file
              after the charset command.  It is possible to include unnamed glyphs  in  the  font
              description  file by using a name of ---; the \N escape sequence is the only way to
              use these.

       \On
       \O[n]  Suppress troff output.  The escapes  \O2,  \O3,  \O4,  and  \O5  are  intended  for
              internal use by grohtml.

              \O0    Disable any ditroff glyphs from being emitted to the device driver, provided
                     that the escape occurs at the outer level (see \O3 and \O4).

              \O1    Enable output of glyphs, provided that the escape occurs at the outer level.

                     \O0 and \O1 also reset the registers \n[opminx], \n[opminy], \n[opmaxx], and
                     \n[opmaxy]  to  -1.  These four registers mark the top left and bottom right
                     hand corners of a box which encompasses all written glyphs.

              \O2    Provided that the escape occurs at the outer level, enable output of  glyphs
                     and also write out to stderr the page number and four registers encompassing
                     the glyphs previously written since the last call to \O.

              \O3    Begin a nesting level.  At start-up, troff  is  at  outer  level.   This  is
                     really  an  internal mechanism for grohtml while producing images.  They are
                     generated by running the troff source through troff to the postscript device
                     and  ghostscript  to  produce images in PNG format.  The \O3 escape starts a
                     new page if the device is not html (to  reduce  the  possibility  of  images
                     crossing a page boundary).

              \O4    End a nesting level.

              \O5[Pfilename]
                     This  escape  is  grohtml specific.  Provided that this escape occurs at the
                     outer nesting level, write filename to stderr.  The position of  the  image,
                     P,  must  be  specified  and  must  be  one  of  l, r, c, or i (left, right,
                     centered, inline).  filename is associated with the production of  the  next
                     inline image.

       \R'name ±n'
              This has the same effect as

                     .nr name ±n

       \s(nn
       \s±(nn Set the point size to nn points; nn must be exactly two digits.

       \s[±n]
       \s±[n]
       \s'±n'
       \s±'n' Set  the  point  size  to n scaled points; n is a numeric expression with a default
              scale indicator of z.

       \Vx
       \V(xx
       \V[xxx]
              Interpolate the contents of the environment variable xxx, as returned by getenv(3).
              \V is interpreted in copy mode.

       \Yx
       \Y(xx
       \Y[xxx]
              This  is  approximately  equivalent  to  \X'\*[xxx]'.   However the contents of the
              string or macro xxx are not interpreted; also it is permitted for xxx to have  been
              defined  as a macro and thus contain newlines (it is not permitted for the argument
              to \X to contain newlines).  The inclusion of newlines requires an extension to the
              UNIX  troff  output  format,  and  confuses  drivers  that  do  not know about this
              extension.

       \Z'anything'
              Print anything and then restore the horizontal and vertical position; anything  may
              not contain tabs or leaders.

       \$0    The  name by which the current macro was invoked.  The als request can make a macro
              have more than one name.

       \$*    In a macro or string, the concatenation of all the arguments separated by spaces.

       \$@    In a macro or string, the concatenation of all the arguments with  each  surrounded
              by double quotes, and separated by spaces.

       \$^    In a macro, the representation of all parameters as if they were an argument to the
              ds request.

       \$(nn
       \$[nnn]
              In a macro or string, this gives the nn-th or nnn-th argument.  Macros and  strings
              can have an unlimited number of arguments.

       \?anything\?
              When  used  in  a  diversion,  this transparently embeds anything in the diversion.
              anything is read  in  copy  mode.   When  the  diversion  is  reread,  anything  is
              interpreted.   anything  may  not  contain  newlines;  use  \! if you want to embed
              newlines in a diversion.  The escape sequence \? is also recognized  in  copy  mode
              and  turned  into a single internal code; it is this code that terminates anything.
              Thus

                     .nr x 1
                     .nf
                     .di d
                     \?\\?\\\\?\\\\\\\\nx\\\\?\\?\?
                     .di
                     .nr x 2
                     .di e
                     .d
                     .di
                     .nr x 3
                     .di f
                     .e
                     .di
                     .nr x 4
                     .f

              prints 4.

       \/     This increases the width of the preceding glyph so that the  spacing  between  that
              glyph  and  the following glyph is correct if the following glyph is a roman glyph.
              It is a good idea  to  use  this  escape  sequence  whenever  an  italic  glyph  is
              immediately followed by a roman glyph without any intervening space.

       \,     This  modifies  the spacing of the following glyph so that the spacing between that
              glyph and the preceding glyph is correct if the preceding glyph is a  roman  glyph.
              It is a good idea to use this escape sequence whenever a roman glyph is immediately
              followed by an italic glyph without any intervening space.

       \)     Like \& except that it behaves like a character declared with the cflags request to
              be transparent for the purposes of end-of-sentence recognition.

       \~     This  produces  an  unbreakable space that stretches like a normal inter-word space
              when a line is adjusted.

       \:     This causes the insertion of a zero-width break point.  It is equal to \% within  a
              word but without insertion of a soft hyphen glyph.

       \#     Everything up to and including the next newline is ignored.  This is interpreted in
              copy mode.  It is like \" except that \" does not ignore the terminating newline.

   New requests
       .aln xx yy
              Create an alias xx for number register object named yy.  The new name and  the  old
              name  are  exactly  equivalent.   If  yy  is  undefined,  a  warning of type reg is
              generated, and the request is ignored.

       .als xx yy
              Create an alias xx for request, string, macro, or diversion object named  yy.   The
              new  name  and  the old name are exactly equivalent (it is similar to a hard rather
              than a soft link).  If yy is undefined, a warning of type mac is generated, and the
              request  is  ignored.   The  de,  am, di, da, ds, and as requests only create a new
              object if the name of the macro, diversion or string is currently undefined  or  if
              it  is  defined  to  be  a  request;  normally they modify the value of an existing
              object.

       .am1 xx yy
              Similar to .am, but compatibility mode is switched off  during  execution.   To  be
              more  precise,  a  ‘compatibility  save’  token is inserted at the beginning of the
              macro addition, and a ‘compatibility restore’ token at the end.  As a  consequence,
              the  requests am, am1, de, and de1 can be intermixed freely since the compatibility
              save/restore tokens only affect the macro parts defined by .am1 and .ds1.

       .ami xx yy
              Append to macro indirectly.  See the dei request below for more information.

       .ami1 xx yy
              Same as the ami request but compatibility mode is switched off during execution.

       .as1 xx yy
              Similar to .as, but compatibility mode is switched off  during  expansion.   To  be
              more  precise,  a  ‘compatibility  save’  token is inserted at the beginning of the
              string, and a ‘compatibility restore’ token at the  end.   As  a  consequence,  the
              requests  as,  as1,  ds,  and  ds1 can be intermixed freely since the compatibility
              save/restore tokens only affect the (sub)strings defined by as1 and ds1.

       .asciify xx
              This request ‘unformats’ the diversion xx in  such  a  way  that  ASCII  and  space
              characters (and some escape sequences) that were formatted and diverted into xx are
              treated like ordinary input characters when xx is reread.  Useful for diversions in
              conjunction  with  the  writem  request.   It can be also used for gross hacks; for
              example, this

                     .tr @.
                     .di x
                     @nr n 1
                     .br
                     .di
                     .tr @@
                     .asciify x
                     .x

              sets register n to 1.  Note that glyph information (font, font size, etc.)  is  not
              preserved; use .unformat instead.

       .backtrace
              Print a backtrace of the input stack on stderr.

       .blm xx
              Set the blank line macro to xx.  If there is a blank line macro, it is invoked when
              a blank line is encountered instead of the usual troff behaviour.

       .box xx
       .boxa xx
              These requests are similar to the di and da requests  with  the  exception  that  a
              partially  filled  line  does not become part of the diversion (i.e., the diversion
              always starts with a  new  line)  but  is  restored  after  ending  the  diversion,
              discarding the partially filled line which possibly comes from the diversion.

       .break Break  out of a while loop.  See also the while and continue requests.  Be sure not
              to confuse this with the br request.

       .brp   This is the same as \p.

       .cflags n c1 c2 ...
              Characters c1, c2, ..., have properties determined by n, which  is  ORed  from  the
              following:

              1      The character ends sentences (initially characters .?! have this property).

              2      Lines  can be broken before the character (initially no characters have this
                     property); a line is not broken at a character with this property unless the
                     characters  on  each side both have non-zero hyphenation codes.  This can be
                     overridden with value 64.

              4      Lines can be broken after the character  (initially  characters  -\[hy]\[em]
                     have  this property); a line is not broken at a character with this property
                     unless the characters on each side both  have  non-zero  hyphenation  codes.
                     This can be overridden with value 64.

              8      The  glyph  associated  with this character overlaps horizontally (initially
                     characters \[ul]\[rn]\[ru]\[radicalex]\[sqrtex] have this property).

              16     The glyph associated with  this  character  overlaps  vertically  (initially
                     glyph \[br] has this property).

              32     An  end-of-sentence character followed by any number of characters with this
                     property is treated as the end of a sentence if followed by a newline or two
                     spaces; in other words the character is transparent for the purposes of end-
                     of-sentence recognition; this is the same as having a zero space  factor  in
                     TeX (initially characters "')]*\[dg]\[rq]\[cq] have this property).

              64     Ignore  hyphenation  code values of the surrounding characters.  Use this in
                     combination  with  values  2  and  4  (initially  no  characters  have  this
                     property).

              128    Prohibit a line break before the character, but allow a line break after the
                     character.  This works only in combination with flags 256 and 512 and has no
                     effect otherwise.

              256    Prohibit a line break after the character, but allow a line break before the
                     character.  This works only in combination with flags 128 and 512 and has no
                     effect otherwise.

              512    Allow  line  break  before  or  after  the  character.   This  works only in
                     combination with flags 128 and 256 and has no effect otherwise.

              Contrary to flag values 2 and 4, the flags 128, 256, and 512  work  pairwise.   If,
              for example, the left character has value 512, and the right character 128, no line
              break gets inserted.  If we use value 6 instead for  the  left  character,  a  line
              break  after  the character can't be suppressed since the right neighbour character
              doesn't get examined.

       .char c string
              [This request can both define characters and glyphs.]

              Define entity c to be string.  To be more precise,  define  (or  even  override)  a
              groff  entity  which  can be accessed with name c on the input side, and which uses
              string on the output side.  Every time glyph c  needs  to  be  printed,  string  is
              processed  in  a  temporary  environment and the result is wrapped up into a single
              object.  Compatibility mode is turned off and the escape  character  is  set  to  \
              while  string  is  being  processed.   Any  emboldening,  constant spacing or track
              kerning is applied to this object rather than to individual glyphs in string.

              A groff object defined by this request  can  be  used  just  like  a  normal  glyph
              provided by the output device.  In particular other characters can be translated to
              it with the tr request; it can be made the leader glyph by the lc request; repeated
              patterns  can  be  drawn with the glyph using the \l and \L escape sequences; words
              containing c can be hyphenated correctly, if the hcode request is used to give  the
              object a hyphenation code.

              There  is  a  special  anti-recursion  feature:  Use  of  glyph  within the glyph's
              definition is handled like normal glyphs not defined with char.

              A glyph definition can be removed with the rchar request.

       .chop xx
              Chop the last element off macro, string, or  diversion  xx.   This  is  useful  for
              removing  the  newline  from  the  end of diversions that are to be interpolated as
              strings.

       .class name c1 c2 ...
              Assign name to a set of characters c1, c2, ..., so that they  can  be  referred  to
              from  other  requests easily (currently .cflags only).  Character ranges (indicated
              by an intermediate ‘-’) and nested classes are possible also.  This  is  useful  to
              assign properties to a large set of characters.

       .close stream
              Close  the  stream named stream; stream will no longer be an acceptable argument to
              the write request.  See the open request.

       .composite glyph1 glyph2
              Map glyph name glyph1 to glyph name glyph2 if it is used in \[...] with  more  than
              one component.

       .continue
              Finish  the  current  iteration  of  a  while  loop.   See also the while and break
              requests.

       .color n
              If n is non-zero or missing, enable colors (this is the default), otherwise disable
              them.

       .cp n  If  n  is non-zero or missing, enable compatibility mode, otherwise disable it.  In
              compatibility mode, long names are not recognized, and the incompatibilities caused
              by long names do not arise.

       .defcolor xxx scheme color_components
              Define  color  xxx.   scheme  can  be  one  of  the  following  values:  rgb (three
              components), cmy (three components), cmyk (four components), and gray or grey  (one
              component).   Color  components  can  be given either as a hexadecimal string or as
              positive decimal integers in the range 0–65535.  A hexadecimal string contains  all
              color  components  concatenated;  it  must  start  with either # or ##.  The former
              specifies hex values in the range 0–255 (which are internally multiplied  by  257),
              the  latter  in  the  range  0–65535.   Examples:  #FFC0CB  (pink),  ##ffff0000ffff
              (magenta).  A new scaling indicator f has  been  introduced  which  multiplies  its
              value  by  65536; this makes it convenient to specify color components as fractions
              in the range 0 to 1.  Example:

                     .defcolor darkgreen rgb 0.1f 0.5f 0.2f

              Note that f is the default scaling indicator for the  defcolor  request,  thus  the
              above statement is equivalent to

                     .defcolor darkgreen rgb 0.1 0.5 0.2

              The  color  named  default  (which  is  device-specific) can't be redefined.  It is
              possible that the default color for \M and \m is not the same.

       .de1 xx yy
              Similar to .de, but compatibility mode is switched off during execution.  On entry,
              the current compatibility mode is saved and restored at exit.

       .dei xx yy
              Define macro indirectly.  The following example

                     .ds xx aa
                     .ds yy bb
                     .dei xx yy

              is equivalent to

                     .de aa bb

       .dei1 xx yy
              Similar to the dei request but compatibility mode is switched off during execution.

       .device anything
              This  is  (almost)  the  same  as  the \X escape.  anything is read in copy mode; a
              leading " is stripped.

       .devicem xx
              This is the same as the \Y escape (to embed  the  contents  of  a  macro  into  the
              intermediate output preceded with ‘x X’).

       .do xxx
              Interpret .xxx with compatibility mode disabled.  For example,

                     .do fam T

              would have the same effect as

                     .fam T

              except  that  it would work even if compatibility mode had been enabled.  Note that
              the previous compatibility mode is restored before any files  sourced  by  xxx  are
              interpreted.

       .ds1 xx yy
              Similar  to  .ds,  but  compatibility mode is switched off during expansion.  To be
              more precise, a ‘compatibility save’ token is inserted  at  the  beginning  of  the
              string, and a ‘compatibility restore’ token at the end.

       .ecs   Save current escape character.

       .ecr   Restore  escape character saved with ecs.  Without a previous call to ecs, ‘\’ will
              be the new escape character.

       .evc xx
              Copy the contents of environment xx to the  current  environment.   No  pushing  or
              popping of environments is done.

       .fam xx
              Set  the current font family to xx.  The current font family is part of the current
              environment.  If xx is missing, switch back to previous font family.  The value  at
              start-up  is  ‘T’.   See the description of the sty request for more information on
              font families.

       .fchar c string
              Define fallback character (or glyph) c to be string.  The syntax of this request is
              the same as the char request; the only difference is that a glyph defined with char
              hides the glyph with the same name in the current font,  whereas  a  glyph  defined
              with fchar is checked only if the particular glyph isn't found in the current font.
              This test happens before checking special fonts.

       .fcolor c
              Set the fill color to c.  If c is missing, switch to the previous fill color.

       .fschar f c string
              Define fallback character (or glyph) c for font f to be string.  The syntax of this
              request is the same as the char request (with an additional argument to specify the
              font); a glyph defined with fschar is searched after the  list  of  fonts  declared
              with the fspecial request but before the list of fonts declared with .special.

       .fspecial f s1 s2 ...
              When  the  current  font  is  f,  fonts s1, s2, ..., are special, that is, they are
              searched for glyphs not in the current font.  Any fonts specified  in  the  special
              request  are  searched  after  fonts  specified  in  the fspecial request.  Without
              argument, reset the list of global special fonts to be empty.

       .ftr f g
              Translate font f to g.  Whenever a font named f is referred  to  in  an  \f  escape
              sequence,  in  the  F  and  S conditional operators, or in the ft, ul, bd, cs, tkf,
              special, fspecial, fp, or sty requests, font g is used.  If g is missing, or  equal
              to f then font f is not translated.

       .fzoom f zoom
              Set  zoom  factor  zoom  for  font f.  zoom must a non-negative integer multiple of
              1/1000th.  If it is missing or is equal to zero, it means the same as 1000,  namely
              no magnification.  f must be a real font name, not a style.

       .gcolor c
              Set the glyph color to c.  If c is missing, switch to the previous glyph color.

       .hcode c1 code1 c2 code2 ...
              Set  the  hyphenation code of character c1 to code1 and that of c2 to code2, and so
              on.  A hyphenation code must be a single input character (not a special  character)
              other  than  a  digit  or  a  space.   Initially  each  lower-case letter a–z has a
              hyphenation code, which is itself, and each upper-case letter A–Z has a hyphenation
              code which is the lower-case version of itself.  See also the hpf request.

       .hla lang
              Set  the  current  hyphenation  language to lang.  Hyphenation exceptions specified
              with the hw request and hyphenation patterns specified with  the  hpf  request  are
              both  associated with the current hyphenation language.  The hla request is usually
              invoked by the troffrc file to set up a default language.

       .hlm n Set the maximum number of consecutive hyphenated lines to n.   If  n  is  negative,
              there  is  no maximum.  The default value is -1.  This value is associated with the
              current environment.  Only lines output  from  an  environment  count  towards  the
              maximum  associated  with that environment.  Hyphens resulting from \% are counted;
              explicit hyphens are not.

       .hpf file
              Read hyphenation patterns from file; this is searched for  in  the  same  way  that
              name.tmac  is searched for when the -mname option is specified.  It should have the
              same format as (simple) TeX  patterns  files.   More  specifically,  the  following
              scanning rules are implemented.

              ·      A percent sign starts a comment (up to the end of the line) even if preceded
                     by a backslash.

              ·      No support for ‘digraphs’ like \$.

              ·      ^^xx (x is 0–9 or a–f) and ^^x (character code of x in the range 0–127)  are
                     recognized; other use of ^ causes an error.

              ·      No macro expansion.

              ·      hpf  checks  for  the  expression  \patterns{...}  (possibly with whitespace
                     before and after the braces).  Everything between the  braces  is  taken  as
                     hyphenation patterns.  Consequently, { and } are not allowed in patterns.

              ·      Similarly, \hyphenation{...} gives a list of hyphenation exceptions.

              ·      \endinput is recognized also.

              ·      For  backwards  compatibility,  if  \patterns  is missing, the whole file is
                     treated as a list of hyphenation patterns (only recognizing the %  character
                     as the start of a comment).

              Use  the  hpfcode request to map the encoding used in hyphenation patterns files to
              groff's input encoding.  By default, everything maps to itself except  letters  ‘A’
              to ‘Z’ which map to ‘a’ to ‘z’.

              The  set of hyphenation patterns is associated with the current language set by the
              hla request.  The hpf request is usually invoked by the troffrc file; a second call
              replaces the old patterns with the new ones.

       .hpfa file
              The  same as hpf except that the hyphenation patterns from file are appended to the
              patterns already loaded in the current language.

       .hpfcode a b c d ...
              After reading a hyphenation patterns file with the hpf or hpfa request, convert all
              characters with character code a in the recently read patterns to character code b,
              character code c to d, etc.  Initially, all character codes map to themselves.  The
              arguments  of hpfcode must be integers in the range 0 to 255.  Note that it is even
              possible to use character codes which are invalid in groff otherwise.

       .hym n Set the hyphenation margin to n: when the current adjustment mode  is  not  b,  the
              line  is  not  hyphenated  if  the  line  is  no  more  than  n short.  The default
              hyphenation margin is 0.  The default scaling indicator for this request is m.  The
              hyphenation  margin  is  associated  with  the  current  environment.   The current
              hyphenation margin is available in the \n[.hym] register.

       .hys n Set the hyphenation space to n:  When  the  current  adjustment  mode  is  b  don't
              hyphenate  the  line  if  the  line can be justified by adding no more than n extra
              space to each word space.  The default hyphenation space is 0.  The default scaling
              indicator  for  this  request  is  m.  The hyphenation space is associated with the
              current environment.  The current hyphenation space is available  in  the  \n[.hys]
              register.

       .itc n macro
              Variant of .it for which a line interrupted with \c counts as one input line.

       .kern n
              If n is non-zero or missing, enable pairwise kerning, otherwise disable it.

       .length xx string
              Compute  the length of string and return it in the number register xx (which is not
              necessarily defined before).

       .linetabs n
              If n is non-zero or missing, enable line-tabs mode, otherwise disable it (which  is
              the  default).   In  line-tabs  mode,  tab  distances  are computed relative to the
              (current) output line.  Otherwise they are taken relative to the input  line.   For
              example, the following

                     .ds x a\t\c
                     .ds y b\t\c
                     .ds z c
                     .ta 1i 3i
                     \*x
                     \*y
                     \*z

              yields

                     a         b         c

              In line-tabs mode, the same code gives

                     a         b                   c

              Line-tabs  mode  is  associated  with the current environment; the read-only number
              register \n[.linetabs] is set to 1 if in line-tabs mode, and 0 otherwise.

       .lsm xx
              Set the leading spaces macro to xx.  If there are leading spaces in an input  line,
              it is invoked instead of the usual troff behaviour; the leading spaces are removed.
              Registers \n[lsn] and \n[lss] hold the number of removed  leading  spaces  and  the
              corresponding horizontal space, respectively.

       .mso file
              The same as the so request except that file is searched for in the same directories
              as macro files for the the -m command line option.  If the file name to be included
              has  the  form name.tmac and it isn't found, mso tries to include tmac.name instead
              and vice versa.  A warning of type file is generated if file can't be  loaded,  and
              the request is ignored.

       .nop anything
              Execute anything.  This is similar to ‘.if 1’.

       .nroff Make the n built-in condition true and the t built-in condition false.  This can be
              reversed using the troff request.

       .open stream filename
              Open filename for writing and associate the stream named stream with it.  See  also
              the close and write requests.

       .opena stream filename
              Like open, but if filename exists, append to it instead of truncating it.

       .output string
              Emit   string   directly   to   the   intermediate  output  (subject  to  copy-mode
              interpretation); this is similar to \! used at the top level.   An  initial  double
              quote in string is stripped off to allow initial blanks.

       .pev   Print the current environment and each defined environment state on stderr.

       .pnr   Print the names and contents of all currently defined number registers on stderr.

       .psbb filename
              Get  the  bounding  box  of a PostScript image filename.  This file must conform to
              Adobe's Document Structuring Conventions; the command  looks  for  a  %%BoundingBox
              comment  to  extract  the  bounding  box  values.   After  a  successful  call, the
              coordinates (in PostScript units) of the lower left and upper right corner  can  be
              found  in  the  registers \n[llx], \n[lly], \n[urx], and \n[ury], respectively.  If
              some error has occurred, the four registers are set to zero.

       .pso command
              This behaves like the so request except that input comes from the  standard  output
              of command.

       .ptr   Print  the  names  and  positions  of all traps (not including input line traps and
              diversion traps) on stderr.  Empty slots in the page trap list are printed as well,
              because they can affect the priority of subsequently planted traps.

       .pvs ±n
              Set the post-vertical line space to n; default scale indicator is p.  This value is
              added to each line after it has been output.  With no argument,  the  post-vertical
              line space is set to its previous value.

              The  total  vertical  line  spacing  consists of four components: .vs and \x with a
              negative value which are applied before the line is output, and .pvs and \x with  a
              positive value which are applied after the line is output.

       .rchar c1 c2 ...
              Remove  the  definitions  of  glyphs  c1,  c2, ... This undoes the effect of a char
              request.

       .return
              Within a macro, return immediately.  If called  with  an  argument,  return  twice,
              namely  from  the  current  macro  and  from the macro one level higher.  No effect
              otherwise.

       .rfschar c1 c2 ...
              Remove the font-specific definitions of glyphs c1, c2, ... This undoes  the  effect
              of a fschar request.

       .rj
       .rj n  Right  justify  the next n input lines.  Without an argument right justify the next
              input line.  The number of lines to be right justified is available in the  \n[.rj]
              register.  This implicitly does .ce 0.  The ce request implicitly does .rj 0.

       .rnn xx yy
              Rename number register xx to yy.

       .schar c string
              Define  global  fallback  character  (or glyph) c to be string.  The syntax of this
              request is the same as the char request; a glyph defined  with  schar  is  searched
              after  the  list  of fonts declared with the special request but before the mounted
              special fonts.

       .shc c Set the soft hyphen character to c.  If c is omitted, the soft hyphen character  is
              set to the default \[hy].  The soft hyphen character is the glyph which is inserted
              when a word is hyphenated at a line break.  If the soft hyphen character  does  not
              exist  in the font of the glyph immediately preceding a potential break point, then
              the line is not broken at that point.  Neither definitions (specified with the char
              request)  nor  translations  (specified  with  the  tr request) are considered when
              finding the soft hyphen character.

       .shift n
              In a macro, shift the arguments by n positions: argument i  becomes  argument  i-n;
              arguments  1  to n are no longer available.  If n is missing, arguments are shifted
              by 1.  Shifting by negative amounts is currently undefined.

       .sizes s1 s2 ... sn [0]
              This command is similar to the sizes command of a DESC file.  It sets the available
              font  sizes  for  the  current  font to s1, s2, ..., sn scaled points.  The list of
              sizes can be terminated by an optional 0.  Each si can also be  a  range  of  sizes
              mn.   Contrary  to  the  font file command, the list can't extend over more than a
              single line.

       .special s1 s2 ...
              Fonts s1, s2, ..., are special and are searched for glyphs not in the current font.
              Without arguments, reset the list of special fonts to be empty.

       .spreadwarn limit
              Make  troff  emit a warning if the additional space inserted for each space between
              words in an output line is larger or equal to limit.  A negative value  is  changed
              to  zero;  no  argument toggles the warning on and off without changing limit.  The
              default scaling indicator is m.  At startup, spreadwarn is deactivated,  and  limit
              is  set  to  3m.   For example, .spreadwarn 0.2m causes a warning if troff must add
              0.2m or more for each interword space in a line.  This request is  active  only  if
              text is justified to both margins (using .ad b).

       .sty n f
              Associate  style  f with font position n.  A font position can be associated either
              with a font or with a style.  The current font is the index of a font position  and
              so is also either a font or a style.  When it is a style, the font that is actually
              used is the font the name of which is the concatenation of the name of the  current
              family  and  the  name of the current style.  For example, if the current font is 1
              and font position 1 is associated with style R and the current font  family  is  T,
              then  font TR is used.  If the current font is not a style, then the current family
              is ignored.  When the requests cs, bd, tkf, uf, or fspecial are applied to a style,
              then  they are applied instead to the member of the current family corresponding to
              that style.  The default family can be set with the -f command  line  option.   The
              styles  command  in  the  DESC  file  controls  which  font  positions (if any) are
              initially associated with styles rather than fonts.

       .substring xx n1 [n2]
              Replace the string named xx with the substring defined by the indices  n1  and  n2.
              The first character in the string has index 0.  If n2 is omitted, it is taken to be
              equal to the string's length.  If the index value n1  or  n2  is  negative,  it  is
              counted  from  the  end  of  the  string,  going  backwards: The last character has
              index -1, the character before the last character has index -2, etc.

       .tkf f s1 n1 s2 n2
              Enable track kerning for font f.  When the current font is f  the  width  of  every
              glyph  is  increased by an amount between n1 and n2; when the current point size is
              less than or equal to s1 the width is increased by n1; when it is greater  than  or
              equal  to  s2  the width is increased by n2; when the point size is greater than or
              equal to s1 and less than or equal to s2 the increase in width is a linear function
              of the point size.

       .tm1 string
              Similar  to the tm request, string is read in copy mode and written on the standard
              error, but an initial double quote in string  is  stripped  off  to  allow  initial
              blanks.

       .tmc string
              Similar to tm1 but without writing a final newline.

       .trf filename
              Transparently  output  the  contents  of  file filename.  Each line is output as if
              preceded by \!; however, the lines are not subject to copy-mode interpretation.  If
              the  file  does  not end with a newline, then a newline is added.  For example, you
              can define a macro x containing the contents of file f, using

                     .di x
                     .trf f
                     .di

              Unlike with the cf request, the file cannot contain characters such as NUL that are
              not valid troff input characters.

       .trin abcd
              This  is  the  same  as  the  tr  request  except that the asciify request uses the
              character code (if any) before the character translation.  Example:

                     .trin ax
                     .di xxx
                     a
                     .br
                     .di
                     .xxx
                     .trin aa
                     .asciify xxx
                     .xxx

              The result is x a.  Using tr, the result would be x x.

       .trnt abcd
              This is the same as the tr request except that the translations  do  not  apply  to
              text that is transparently throughput into a diversion with \!.  For example,

                     .tr ab
                     .di x
                     \!.tm a
                     .di
                     .x

              prints b; if trnt is used instead of tr it prints a.

       .troff Make  the  n  built-in  condition  false,  and the t built-in condition true.  This
              undoes the effect of the nroff request.

       .unformat xx
              This request ‘unformats’ the diversion xx.  Contrary to the asciify request,  which
              tries  to  convert formatted elements of the diversion back to input tokens as much
              as possible, .unformat only handles tabs and spaces between words  (usually  caused
              by  spaces  or newlines in the input) specially.  The former are treated as if they
              were input tokens, and the latter are stretchable again.  Note  that  the  vertical
              size  of  lines is not preserved.  Glyph information (font, font size, space width,
              etc.) is retained.  Useful in conjunction with the box and boxa requests.

       .vpt n Enable vertical position traps if n is non-zero, disable them otherwise.   Vertical
              position traps are traps set by the wh or dt requests.  Traps set by the it request
              are not vertical position traps.  The  parameter  that  controls  whether  vertical
              position  traps  are  enabled  is  global.   Initially  vertical position traps are
              enabled.

       .warn n
              Control warnings.  n is the sum of the numbers associated with each warning that is
              to  be  enabled;  all other warnings are disabled.  The number associated with each
              warning is listed in troff(1).  For example, .warn 0  disables  all  warnings,  and
              .warn 1 disables all warnings except that about missing glyphs.  If n is not given,
              all warnings are enabled.

       .warnscale si
              Set the scaling indicator used in warnings to si.  Valid values for si are u, i, c,
              p, and P.  At startup, it is set to i.

       .while c anything
              While  condition  c  is  true,  accept  anything  as  input; c can be any condition
              acceptable to an if request; anything can comprise multiple lines if the first line
              starts  with  \{  and  the last line ends with \}.  See also the break and continue
              requests.

       .write stream anything
              Write anything to the stream named stream.  stream must previously  have  been  the
              subject  of  an  open  request.   anything  is  read  in  copy mode; a leading " is
              stripped.

       .writec stream anything
              Similar to write but without writing a final newline.

       .writem stream xx
              Write the contents of the macro or string xx to the stream  named  stream.   stream
              must previously have been the subject of an open request.  xx is read in copy mode.

   Extended escape sequences
       \D'...'
              All  drawing  commands of groff's intermediate output are accepted.  See subsection
              Drawing Commands below for more information.

   Extended requests
       .cf filename
              When used in a diversion, this embeds  in  the  diversion  an  object  which,  when
              reread,  will  cause the contents of filename to be transparently copied through to
              the output.  In UNIX troff, the contents of filename is immediately copied  through
              to the output regardless of whether there is a current diversion; this behaviour is
              so anomalous that it must be considered a bug.

       .de xx yy
       .am xx yy
       .ds xx yy
       .as xx yy
              In compatibility mode, these requests behaves similar  to  .de1,  .am1,  .ds1,  and
              .as1,  respectively: A ‘compatibility save’ token is inserted at the beginning, and
              a ‘compatibility restore’ token at the end, with  compatibility  mode  switched  on
              during execution.

       .ev xx If  xx  is  not  a  number,  this  switches  to a named environment called xx.  The
              environment should be popped with a matching ev request without any arguments, just
              as  for  numbered  environments.   There  is  no  limit  on  the  number  of  named
              environments; they are created the first time that they are referenced.

       .ss m n
              When two arguments are given to the ss  request,  the  second  argument  gives  the
              sentence  space size.  If the second argument is not given, the sentence space size
              is the same as the word space size.  Like the word space size, the  sentence  space
              is  in  units  of  one  twelfth  of  the spacewidth parameter for the current font.
              Initially both the word space size and the sentence space size are 12.  Contrary to
              UNIX  troff,  GNU  troff  handles this request in nroff mode also; a given value is
              then rounded down to the nearest multiple of 12.  The sentence space size  is  used
              in two circumstances.  If the end of a sentence occurs at the end of a line in fill
              mode, then both an inter-word space and a sentence space are added; if  two  spaces
              follow  the  end  of a sentence in the middle of a line, then the second space is a
              sentence space.  Note that the behaviour of UNIX troff are exactly  that  exhibited
              by  GNU troff if a second argument is never given to the ss request.  In GNU troff,
              as in UNIX troff, you should always follow a sentence with either a newline or  two
              spaces.

       .ta n1 n2 ... nn T r1 r2 ... rn
              Set tabs at positions n1, n2, ..., nn and then set tabs at nn+r1, nn+r2, ..., nn+rn
              and then at nn+rn+r1, nn+rn+r2, ..., nn+rn+rn, and so on.  For example,

                     .ta T .5i

              sets tabs every half an inch.

   New number registers
       The following read-only registers are available:

       \n[.br]
              Within a macro call, it is set to 1 if  the  macro  is  called  with  the  ‘normal’
              control  character  (‘.’  by  default),  and  set  to  0 otherwise.  This allows to
              reliably modify requests.

                     .als bp*orig bp
                     .de bp
                     .tm before bp
                     .ie \\n[.br] .bp*orig
                     .el 'bp*orig
                     .tm after bp
                     ..

              Using this register outside of a macro makes no sense (it always  returns  zero  in
              such cases).

       \n[.C] 1 if compatibility mode is in effect, 0 otherwise.

       \n[.cdp]
              The  depth  of  the last glyph added to the current environment.  It is positive if
              the glyph extends below the baseline.

       \n[.ce]
              The number of lines remaining to be centered, as set by the ce request.

       \n[.cht]
              The height of the last glyph added to the current environment.  It is  positive  if
              the glyph extends above the baseline.

       \n[.color]
              1 if colors are enabled, 0 otherwise.

       \n[.csk]
              The  skew  of the last glyph added to the current environment.  The skew of a glyph
              is how far to the right of the center of a glyph the center of an accent over  that
              glyph should be placed.

       \n[.ev]
              The name or number of the current environment.  This is a string-valued register.

       \n[.fam]
              The current font family.  This is a string-valued register.

       \n[.fn]
              The  current  (internal) real font name.  This is a string-valued register.  If the
              current font is a style, the value of \n[.fn] is the proper concatenation of family
              and style name.

       \n[.fp]
              The number of the next free font position.

       \n[.g] Always  1.   Macros should use this to determine whether they are running under GNU
              troff.

       \n[.height]
              The current height of the font as set with \H.

       \n[.hla]
              The current hyphenation language as set by the hla request.

       \n[.hlc]
              The number of immediately preceding consecutive hyphenated lines.

       \n[.hlm]
              The maximum allowed number of consecutive hyphenated  lines,  as  set  by  the  hlm
              request.

       \n[.hy]
              The current hyphenation flags (as set by the hy request).

       \n[.hym]
              The current hyphenation margin (as set by the hym request).

       \n[.hys]
              The current hyphenation space (as set by the hys request).

       \n[.in]
              The indentation that applies to the current output line.

       \n[.int]
              Set  to  a  positive value if last output line is interrupted (i.e., if it contains
              \c).

       \n[.kern]
              1 if pairwise kerning is enabled, 0 otherwise.

       \n[.lg]
              The current ligature mode (as set by the lg request).

       \n[.linetabs]
              The current line-tabs mode (as set by the linetabs request).

       \n[.ll]
              The line length that applies to the current output line.

       \n[.lt]
              The title length as set by the lt request.

       \n[.m] The name of the current drawing color.  This is a string-valued register.

       \n[.M] The name of the current background color.  This is a string-valued register.

       \n[.ne]
              The amount of space that was needed in the last ne request that caused a trap to be
              sprung.  Useful in conjunction with the \n[.trunc] register.

       \n[.ns]
              1 if no-space mode is active, 0 otherwise.

       \n[.O] The current output level as set with \O.

       \n[.P] 1 if the current page is in the output list set with -o.

       \n[.pe]
              1 during a page ejection caused by the bp request, 0 otherwise.

       \n[.pn]
              The number of the next page, either the value set by a pn request, or the number of
              the current page plus 1.

       \n[.ps]
              The current point size in scaled points.

       \n[.psr]
              The last-requested point size in scaled points.

       \n[.pvs]
              The current post-vertical line space as set with the pvs request.

       \n[.rj]
              The number of lines to be right-justified as set by the rj request.

       \n[.slant]
              The slant of the current font as set with \S.

       \n[.sr]
              The last requested point size in points as a decimal fraction.  This is  a  string-
              valued register.

       \n[.ss]
       \n[.sss]
              These  give  the  values of the parameters set by the first and second arguments of
              the ss request.

       \n[.sty]
              The current font style.  This is a string-valued register.

       \n[.tabs]
              A string representation of the current tab settings suitable for use as an argument
              to the ta request.

       \n[.trunc]
              The  amount  of  vertical  space  truncated  by  the  most recently sprung vertical
              position trap, or, if the trap was sprung by a ne  request,  minus  the  amount  of
              vertical motion produced by the ne request.  In other words, at the point a trap is
              sprung, it represents the difference of what the vertical position would have  been
              but  for  the  trap,  and  what  the  vertical  position  actually  is.   Useful in
              conjunction with the \n[.ne] register.

       \n[.U] Set to 1 if in safer mode and to 0 if in unsafe mode (as given with the -U  command
              line option).

       \n[.vpt]
              1 if vertical position traps are enabled, 0 otherwise.

       \n[.warn]
              The sum of the numbers associated with each of the currently enabled warnings.  The
              number associated with each warning is listed in troff(1).

       \n[.x] The major version number.  For example, if the version number is 1.03, then  \n[.x]
              contains 1.

       \n[.y] The  minor version number.  For example, if the version number is 1.03, then \n[.y]
              contains 03.

       \n[.Y] The revision number of groff.

       \n[.zoom]
              The zoom value of  the  current  font,  in  multiples  of  1/1000th.   Zero  if  no
              magnification.

       \n[llx]
       \n[lly]
       \n[urx]
       \n[ury]
              These  four  read/write  registers  are  set  by  the  psbb request and contain the
              bounding box values (in PostScript units) of a given PostScript image.

       The following read/write registers are set by the \w escape sequence:

       \n[rst]
       \n[rsb]
              Like the st and sb registers, but take account of the heights and depths of glyphs.

       \n[ssc]
              The amount of horizontal space (possibly negative) that should be added to the last
              glyph before a subscript.

       \n[skw]
              How  far to right of the center of the last glyph in the \w argument, the center of
              an accent from a roman font should be placed over that glyph.

       Other available read/write number registers are:

       \n[c.] The current input line number.  \n[.c] is a read-only alias to this register.

       \n[hours]
              The number of hours past midnight.  Initialized at start-up.

       \n[hp] The current horizontal position at input line.

       \n[lsn]
       \n[lss]
              If there are leading spaces in an input line, these registers hold  the  number  of
              leading spaces and the corresponding horizontal space, respectively.

       \n[minutes]
              The number of minutes after the hour.  Initialized at start-up.

       \n[seconds]
              The number of seconds after the minute.  Initialized at start-up.

       \n[systat]
              The return value of the system() function executed by the last sy request.

       \n[slimit]
              If  greater than 0, the maximum number of objects on the input stack.  If less than
              or equal to 0, there is no limit on the number of objects on the input stack.  With
              no limit, recursion can continue until virtual memory is exhausted.

       \n[year]
              The  current  year.   Note that the traditional troff number register \n[yr] is the
              current year minus 1900.

   Miscellaneous
       troff predefines a single (read/write) string-based register, \*[.T], which  contains  the
       argument  given  to  the  -T  command  line  option, namely the current output device (for
       example, latin1 or ascii).  Note that this is not  the  same  as  the  (read-only)  number
       register  \n[.T]  which  is  defined  to  be 1 if troff is called with the -T command line
       option, and zero otherwise.  This behaviour is different to UNIX troff.

       Fonts not listed in the DESC file are automatically mounted on  the  next  available  font
       position  when  they  are  referenced.   If a font is to be mounted explicitly with the fp
       request on an unused font position,  it  should  be  mounted  on  the  first  unused  font
       position, which can be found in the \n[.fp] register; although troff does not enforce this
       strictly, it does not allow a font to be mounted  at  a  position  whose  number  is  much
       greater than that of any currently used position.

       Interpolating  a  string  does not hide existing macro arguments.  Thus in a macro, a more
       efficient way of doing

              .xx \\$@

       is

              \\*[xx]\\

       If the font description file contains pairwise kerning information, glyphs from that  font
       are kerned.  Kerning between two glyphs can be inhibited by placing a \& between them.

       In a string comparison in a condition, characters that appear at different input levels to
       the first delimiter character are not recognized as the second or third delimiters.   This
       applies  also  to  the tl request.  In a \w escape sequence, a character that appears at a
       different input level to the starting delimiter character is not recognized as the closing
       delimiter  character.   The same is true for \A, \b, \B, \C, \l, \L, \o, \X, and \Z.  When
       decoding a macro or string argument that is delimited by double quotes, a  character  that
       appears  at  a different input level to the starting delimiter character is not recognized
       as the closing delimiter character.  The implementation of \$@  ensures  that  the  double
       quotes  surrounding  an argument appear at the same input level, which is different to the
       input level of the argument itself.  In a long escape  name  ]  is  not  recognized  as  a
       closing  delimiter  except  when  it  occurs at the same input level as the opening [.  In
       compatibility mode, no attention is paid to the input-level.

       There are some new types of condition:

       .if rxxx
              True if there is a number register named xxx.

       .if dxxx
              True if there is a string, macro, diversion, or request named xxx.

       .if mxxx
              True if there is a color named xxx.

       .if cch
              True if there is a character (or  glyph)  ch  available;  ch  is  either  an  ASCII
              character  or a glyph (special character) \N'xxx', \(xx or \[xxx]; the condition is
              also true if ch has been defined by the char request.

       .if Ff True if font f exists.  f is handled as if it was opened with the ft request  (this
              is, font translation and styles are applied), without actually mounting it.

       .if Ss True if style s has been registered.  Font translation is applied.

       The tr request can now map characters onto \~.

       The  space width emitted by the \| and \^ escape sequences can be controlled on a per-font
       basis.  If there is a glyph named \| or \^, respectively  (note  the  leading  backslash),
       defined in the current font file, use this glyph's width instead of the default value.

       It is now possible to have whitespace between the first and second dot (or the name of the
       ending macro) to end a macro definition.  Example:

              .if t \{\
              .  de bar
              .    nop Hello, I'm ‘bar’.
              .  .
              .\}

INTERMEDIATE OUTPUT FORMAT

       This section describes the format output by GNU troff.  The  output  format  used  by  GNU
       troff is very similar to that used by Unix device-independent troff.  Only the differences
       are documented here.

   Units
       The argument to the s command is in scaled points (units  of  points/n,  where  n  is  the
       argument  to  the  sizescale  command   in  the  DESC file).  The argument to the x Height
       command is also in scaled points.

   Text Commands
       Nn     Print glyph with index n (a non-negative integer) of the current font.

       If the tcommand line is present in the DESC file, troff uses the following two commands.

       txxx   xxx is any sequence of characters terminated by a space or a newline  (to  be  more
              precise,  it  is  a  sequence  of  glyphs which are accessed with the corresponding
              characters); the first character should be printed at  the  current  position,  the
              current  horizontal  position  should  be  increased  by  the  width  of  the first
              character, and so on for each character.  The width of the glyph is that  given  in
              the font file, appropriately scaled for the current point size, and rounded so that
              it is a multiple of  the  horizontal  resolution.   Special  characters  cannot  be
              printed using this command.

       un xxx This  is  same  as  the  t  command  except that after printing each character, the
              current horizontal position is increased by the sum of the width of that  character
              and n.

       Note  that  single  characters  can have the eighth bit set, as can the names of fonts and
       special characters.

       The names of glyphs and fonts can be of arbitrary length; drivers should not  assume  that
       they are only two characters long.

       When  a  glyph is to be printed, that glyph is always in the current font.  Unlike device-
       independent troff, it is not necessary for drivers to  search  special  fonts  to  find  a
       glyph.

       For color support, some new commands have been added:

       mc cyan magenta yellow
       md
       mg gray
       mk cyan magenta yellow black
       mr red green blue
              Set the color components of the current drawing color, using various color schemes.
              md resets the drawing color to the default value.  The arguments  are  integers  in
              the range 0 to 65536.

       The x device control command has been extended.

       x u n  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.

   Drawing Commands
       The D drawing command has been extended.  These extensions are not used by GNU pic if  the
       -n option is given.

       Df n\n Set  the  shade  of  gray  to  be used for filling solid objects to n; n must be an
              integer between 0 and 1000, where 0 corresponds  solid  white  and  1000  to  solid
              black,  and  values  in  between  correspond  to intermediate shades of gray.  This
              applies only to solid circles, solid ellipses and solid polygons.   By  default,  a
              level  of  1000  is  used.  Whatever color a solid object has, it should completely
              obscure everything beneath it.  A value greater than 1000 or less than 0  can  also
              be  used:  this  means fill with the shade of gray that is currently being used for
              lines and text.  Normally this is black, but some drivers  may  provide  a  way  of
              changing this.

              The  corresponding  \D'f...' command shouldn't be used since its argument is always
              rounded to an integer multiple of the  horizontal  resolution  which  can  lead  to
              surprising results.

       DC d\n Draw  a  solid  circle  with a diameter of d with the leftmost point at the current
              position.

       DE dx dy\n
              Draw a solid ellipse with a horizontal diameter of dx and a vertical diameter of dy
              with the leftmost point at the current position.  delim $$

       Dp $dx sub 1$ $dy sub 1$ $dx sub 2$ $dy sub 2$ $...$ $dx sub n$ $dy sub n$\n
              Draw a polygon with, for $i = 1 ,..., n+1$, the i-th vertex at the current position
              $+ sum from j=1 to i-1 ( dx sub j , dy sub j )$.  At the moment, GNU pic only  uses
              this command to generate triangles and rectangles.

       DP $dx sub 1$ $dy sub 1$ $dx sub 2$ $dy sub 2$ $...$ $dx sub n$ $dy sub n$\n
              Like Dp but draw a solid rather than outlined polygon.

       Dt n\n Set  the  current  line  thickness  to  n  machine units.  Traditionally Unix troff
              drivers use a line thickness proportional to the current point size; drivers should
              continue  to  do  this if no Dt command has been given, or if a Dt command has been
              given with a negative value of n.  A zero value of n selects the smallest available
              line thickness.

       A  difficulty  arises in how the current position should be changed after the execution of
       these commands.  This is not of great importance since the code generated by GNU pic  does
       not depend on this.  Given a drawing command of the form

              \D'c $x sub 1$ $y sub 1$ $x sub 2$ $y sub 2$ $...$ $x sub n$ $y sub n$'

       where  c  is  not  one  of  c, e, l, a, or ~, Unix troff treats each of the $x sub i$ as a
       horizontal quantity, and each of the $y sub i$ as a vertical quantity and assumes that the
       width of the drawn object is $sum from i=1 to n x sub i$, and that the height is $sum from
       i=1 to n y sub i$.  (The assumption about the height can be seen by examining the  st  and
       sb  registers after using such a D command in a \w escape sequence).  This rule also holds
       for all the original drawing  commands  with  the  exception  of  De.   For  the  sake  of
       compatibility  GNU troff also follows this rule, even though it produces an ugly result in
       the case of the Dt and Df, and, to a lesser extent, DE commands.  Thus after  executing  a
       D command of the form

              Dc $x sub 1$ $y sub 1$ $x sub 2$ $y sub 2$ $...$ $x sub n$ $y sub n$\n

       the current position should be increased by $( sum from i=1 to n x sub i , sum from i=1 to
       n y sub i )$.

       Another set of extensions is

       DFc cyan magenta yellow\n
       DFd\n
       DFg gray\n
       DFk cyan magenta yellow black\n
       DFr red green blue\n
              Set the color components of the filling color similar to the m commands above.

       The current position isn't changed by those colour commands (contrary to Df).

   Device Control Commands
       There is a continuation convention which permits  the  argument  to  the  x X  command  to
       contain  newlines: when outputting the argument to the x X command, GNU troff follows each
       newline in the argument with a + character (as usual, it terminates  the  entire  argument
       with a newline); thus if the line after the line containing the x X command starts with +,
       then the newline ending the line containing the x X command should be treated as  part  of
       the  argument  to  the  x X  command,  the  +  should be ignored, and the part of the line
       following the + should be treated like the part of the line following the x X command.

       The first three output commands are guaranteed to be:

              x T device
              x res n h v
              x init

INCOMPATIBILITIES

       In spite of the many extensions, groff has retained compatibility to classical troff to  a
       large  degree.   For  the  cases  where  the  extensions  lead  to  collisions,  a special
       compatibility mode with the restricted, old functionality was created for groff.

   Groff Language
       groff provides a compatibility mode that allows to process roff code written for classical
       troff or for other implementations of roff in a consistent way.

       Compatibility  mode can be turned on with the -C command line option, and turned on or off
       with the .cp request.  The number register  \n(.C  is  1  if  compatibility  mode  is  on,
       0 otherwise.

       This   became   necessary   because   the   GNU   concept   for  long  names  causes  some
       incompatibilities.  Classical troff interprets

              .dsabcd

       as defining a string ab with contents cd.  In groff mode, this is considered as a call  of
       a macro named dsabcd.

       Also  classical  troff  interprets \*[ or \n[ as references to a string or number register
       called [ while groff takes this as the start of a long name.

       In compatibility mode, groff interprets these things in the traditional way; so long names
       are not recognized.

       On  the  other  hand,  groff in GNU native mode does not allow to use the single-character
       escapes \\ (backslash), \| (vertical bar), \^ (caret), \& (ampersand), \{ (opening brace),
       \}  (closing  brace),  ‘\ ’  (space),  \'  (single  quote), \` (backquote), \- (minus), \_
       (underline), \! (bang), \% (percent), and \c (character c) in names  of  strings,  macros,
       diversions, number registers, fonts or environments, whereas classical troff does.

       The \A escape sequence can be helpful in avoiding these escape sequences in names.

       Fractional  point  sizes cause one noteworthy incompatibility.  In classical troff, the ps
       request ignores scale indicators and so

              .ps 10u

       sets the point size to 10 points, whereas in groff native mode the point size  is  set  to
       10 scaled points.

       In  groff,  there  is  a  fundamental difference between unformatted input characters, and
       formatted output characters (glyphs).  Everything that affects how a glyph  is  output  is
       stored  with  the  glyph;  once  a  glyph  has  been  constructed  it is unaffected by any
       subsequent requests that are executed, including the bd, cs, tkf, tr, or fp requests.

       Normally glyphs are constructed from input characters at the moment immediately before the
       glyph  is  added  to  the current output line.  Macros, diversions and strings are all, in
       fact, the same type of object; they contain lists of input characters and  glyphs  in  any
       combination.

       Special  characters  can  be  both;  before  being  added to the output, they act as input
       entities, afterwards they denote glyphs.

       A glyph does not behave like an input character for the purposes of macro  processing;  it
       does  not inherit any of the special properties that the input character from which it was
       constructed might have had.  The following example makes things clearer.

              .di x
              \\\\
              .br
              .di
              .x

       With GNU troff this is printed as \\.  So each pair of input backslashes  ‘\\’  is  turned
       into  a  single  output  backslash  glyph ‘\’ and the resulting output backslashes are not
       interpreted as escape characters when they are reread.

       Classical troff would interpret them as escape characters when they were reread and  would
       end up printing a single backslash ‘\’.

       In  GNU,  the correct way to get a printable version of the backslash character ’\’ is the
       \(rs escape sequence, but classical troff does not provide a clean feature for  getting  a
       non-syntactical  backslash.  A close method is the printable version of the current escape
       character using the \e escape sequence; this works if the current escape character is  not
       redefined.   It  works  in  both  GNU mode and compatibility mode, while dirty tricks like
       specifying a sequence of multiple backslashes do not  work  reliably;  for  the  different
       handling in diversions, macro definitions, or text mode quickly leads to a confusion about
       the necessary number of backslashes.

       To store an escape sequence in a diversion that  is  interpreted  when  the  diversion  is
       reread,  either  the  traditional  \!  transparent  output  facility  or the new \? escape
       sequence can be used.

   Intermediate Output
       The groff intermediate output format is in a state of  evolution.   So  far  it  has  some
       incompatibilities,  but  it is intended to establish a full compatibility to the classical
       troff output format.  Actually the following incompatibilities exist:

       · The positioning  after  the  drawing  of  the  polygons  conflicts  with  the  classical
         definition.

       · The  intermediate  output  cannot  be  rescaled  to  other devices as classical ‘device-
         independent’ troff did.

SEE ALSO

       The groff info file,  cf.  info(1)  presents  all  groff  documentation  within  a  single
       document.

       groff(1)
              A list of all documentation around groff.

       groff(7)
              A  description  of the groff language, including a short, but complete reference of
              all predefined requests, registers, and escapes of plain groff.  From  the  command
              line, this is called using

                     man 7 groff

       roff(7)
              A survey of roff systems, including pointers to further historical documentation.

       [CSTR #54]
              The  Nroff/Troff  User's  Manual  by J. F. Ossanna of 1976 in the revision of Brian
              Kernighan  of  1992,  being  the  classical  troff  documentation  ⟨http://cm.bell-
              labs.com/cm/cs/cstr/54.ps.gz⟩.

COPYING

       Copyright © 1989-2014 Free Software Foundation, Inc.

       This  file  is  part  of groff, the GNU roff type-setting system.  It is the source of the
       man-page groff_diff(7).

       Permission is granted to copy, distribute and/or modify this document under the  terms  of
       the GNU Free Documentation License, Version 1.3 or any later version published by the Free
       Software Foundation; with no Front-Cover Texts, and with no Back-Cover Texts.

       A copy of the Free Documentation License is included as a file  called  FDL  in  the  main
       directory  of  the  groff  source package, it is also available in the internet at GNU FDL
       license ⟨http://www.gnu.org/copyleft/fdl.html⟩.

AUTHORS

       This document was written by James Clark ⟨jjc@jclark.com⟩, was modified by Werner  Lemberg
       ⟨wl@gnu.org⟩ and Bernd Warken ⟨groff-bernd.warken-72@web.de⟩.