Provided by: groff_1.18.1.1-11_i386 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 Users
       Manual by Osanna and Kernighan.  This inludes 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.

       At  the  moment,  this  document  is  the  place  of  the  most  actual
       documentation  within  the  groff  system.   This  might  change in the
       future.  Actually, all  novelties  of  the  groff  language  are  first
       described  here  and  will  pervade  into the other documents only at a
       later stage.

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, and  colors  can  be  of  any  length.   In  escape
       sequences,  additionally  to  the  classical (xx construction for a two
       character name, you can use [xxx] for a name of arbitrary  length,  for
       example in

       \[xxx]    Print the special character called xxx.

       \f[xxx]   Set  font  xxx.   Additionally, \f[] is a new syntax 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 pointsizes
       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
       pointsize  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  will  be
       equivalent  to  a  millipoint;  the  call  .ps 10.25  is  equivalent to
       .ps 10.25z and so sets the pointsize to 10250 scaled points,  which  is
       equal to 10.25 points.

       The  number  register \n[.s] returns the pointsize in points as decimal
       fraction.  There is also a new number register \n[.ps] that returns the
       pointsize 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
       \Aanything’
              This expands to 1 or 0 resp., depending on whether  anything  is
              or  is not acceptable as the name of a string, macro, diversion,
              number register, environment, font, or color.  It will return  0
              if anything is empty.  This is useful if you want to lookup user
              input in some sort of associative table.

       \Banything’
              This expands to 1 or 0 resp., depending on whether  anything  is
              or  is  not  a  valid  numeric  expression.  It will return 0 if
              anything is empty.

       \Cxxx’
              Typeset character 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 will 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 color (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.

       \Nn’  Typeset the character with code n in the current font.  n can be
              any  integer.   Most  devices  only  have  characters with codes
              between 0 and 255.  If the  current  font  does  not  contain  a
              character  with  that  code, special fonts will not be searched.
              The \N escape sequence can be conveniently used  in  conjunction
              with the char request, for example

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

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

       \On
       \O[n]  Suppressing  troff  output.   The escapes \02, \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 will start 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 will be associated with  the
                     production of the next inline image.

       \Rname 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 will confuse
              drivers that do not know about this extension.

       \Zanything’
              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.

       \$(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 will transparently embed anything
              in  the  diversion.   anything  is  read in copy mode.  When the
              diversion is reread, anything will be interpreted.  anything may
              not  contain newlines; use \! if you want to embed newlines in a
              diversion.  The escape sequence \? is also  recognised  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

              will print 4.

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

       \,     This modifies the spacing of the following character so that the
              spacing between that character and the preceding character  will
              correct  if the preceding character is a roman character.  It is
              a good idea  to  use  this  escape  sequence  whenever  a  roman
              character is immediately followed by an italic character 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
              character.

       \#     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 will be  exactly  equivalent.   If  yy  is
              undefined,  a  warning  of  type  reg will be generated, and the
              request will be ignored.

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

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

       .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.

       .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 will be 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

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

       .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.

       .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 will be 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 will not become  part  of
              the  diversion  (i.e.,  the  diversion  always starts with a new
              line) but 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 will not be broken
                     at a character with this property unless  the  characters
                     on each side both have non-zero hyphenation codes.

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

              8      The character overlaps horizontally (initially characters
                     \(ul\(rn\(ru have this property).

              16     The character overlaps  vertically  (initially  character
                     \(br has this property).

              32     An  end-of-sentence  character  followed by any number of
                     characters with this property will be 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 have this property).

       .char c string
              Define  character  c to be string.  Every time character c needs
              to  be  printed,  string  will  be  processed  in  a   temporary
              environment  and  the  result  will  be wrapped up into a single
              object.  Compatibility mode will be turned off  and  the  escape
              character will be set to \ while string is being processed.  Any
              emboldening, constant spacing or track kerning will  be  applied
              to this object rather than to individual characters in string.

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

              There is a special  anti-recursion  feature:  use  of  character
              within  the  character’s  definition will be handled like normal
              characters not defined with char.

              A character definition can be removed with the rchar request.

       .chop xx
              Chop the last character 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.

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

       .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
              recognised, and the incompatibilities caused by  long  names  do
              not arise.

       .defcolor xxx scheme color_components
              Define  color.   scheme  can be one of the following values: rgb
              (three  components),  cym   (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.

       .dei xx yy
              Define macro indirectly.  The following example

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

              is equivalent to

                     .de aa bb

       .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.

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

       .fspecial f s1 s2...
              When  the  current  font is f, fonts s1, s2,... will be special,
              that is, they will searched for characters not  in  the  current
              font.   Any  fonts  specified  in  the  special  request will be
              searched after fonts specified in the fspecial request.

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

       .hcode c1 code1 c2 code2...
              Set the hyphenation code of character c1 to code1 and that of c2
              to  code2.   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.

       .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 will be 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.

              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 will not be 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.

       .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.

       .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.

       .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 will be 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 characters c1, c2,...  This undoes the
              effect of a char request.

       .return
              Within a macro, return immediately.  No effect otherwise.

       .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.

       .shc c Set the soft hyphen character to c.  If c is omitted,  the  soft
              hyphen  character  will  be  set  to the default \(hy.  The soft
              hyphen character is the character which will be inserted when  a
              word  is  hyphenated  at  a  line  break.   If  the  soft hyphen
              character  does  not  exist  in  the  font  of   the   character
              immediately  preceding  a  potential  break point, then the line
              will  not  be  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  will  no  longer  be
              available.   If  n  is  missing, arguments will be 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 m-n.
              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 will be searched for characters
              not in the current font.

       .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 will
              cause 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 will be 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  will  instead
              be  applied to the member of the current family corresponding to
              that style.  The default family can be set with the  -f  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
              will be 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 character will be increased by an amount  between
              n1  and n2; when the current point size is less than or equal to
              s1 the width will be increased by n1; when it is greater than or
              equal  to  s2  the width will be 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 will be 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 legal troff input characters.

       .trin abcd
              This is the same as the  tr  request  except  that  the  asciify
              request  will  use  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

              will print b; if trnt is used instead of tr it will print 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  will  only  handle  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  will  be
              disabled.   The number associated with each warning is listed in
              troff(1).  For example, .warn 0 will disable all  warnings,  and
              .warn  1  will  disable  all  warnings except that about missing
              characters.  If n is not given, all warnings will be 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 " will be 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 requests
       .cf filename
              When used in a diversion, this will embed 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.

       .ev xx If xx is not a number, this will switch 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 will be 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 will be 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 will be  added;  if  two  spaces
              follow  the  end of a sentence in the middle of a line, then the
              second space will be a sentence space.  Note that the  behaviour
              of  UNIX  troff will be 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

              will set tabs every half an inch.

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

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

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

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

       \n[.csk]
              The skew of the last character added to the current environment.
              The skew of a character is how far to the right of the center of
              a  character  the center of an accent over that character 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[.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 indent 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[.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[.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 pointsize in scaled points.

       \n[.psr]
              The last-requested pointsize 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[.sr]
              The last requested pointsize 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[.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[.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] will contain 1.

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

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

       \n[llx]
       \n[lly]
       \n[urx]
       \n[ury]
              These  four  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 characters.

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

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

       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[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  will  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,
       characters  from  that  font  will  be  kerned.   Kerning  between  two
       characters 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  will  not  be
       recognised 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 will not be
       recognised 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
       will not  be  recognised  as  the  closing  delimiter  character.   The
       implementation  of  \$@  ensures  that the double quotes surrounding an
       argument will appear the same input level, which will be  different  to
       the  input  level of the argument itself.  In a long escape name ] will
       not be 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 ch available; ch is either an ASCII
              character or a special character \(xx or \[xxx];  the  condition
              will also be true if ch has been defined by the char request.

       The tr request can now map characters onto \~.

       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:

              .de foo
              . nop Hello, Imfoo.
              . nop I will now definebar.
              . de bar
              . nop Hello, Imbar.
              . .
              . nop Done.
              ..
              .foo
              .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 character with index n (a  non-negative  integer)  of  the
              current font.

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

       txxx   xxx is any sequence of characters terminated by  a  space  or  a
              newline;  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 character 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 characters and fonts can be of arbitrary  length;  drivers
       should not assume that they will be only two characters long.

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

       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 will not  be
       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 will be 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 will be black, but  some  drivers
              may provide a way of changing this.

       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

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

       where c is not one of c, e, l, a, or ~, Unix troff will treat  each  of
       the  $x sub i$ as a horizontal quantity, and each of the $y sub i$ as a
       vertical quantity and will assume 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, 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.

       Note  that  Df  is  now  mapped  onto  DFg.  The current position isn’t
       changed by those colour commands.

   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 will follow each newline in the argument with  a
       +  character  (as  usual,  it will terminate 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  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 will be
       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  pointsizes  cause  one  noteworthy   incompatibility.    In
       classical troff, the ps request ignores scale indicators and so

              .ps 10u

       will  set  the pointsize to 10 points, whereas in groff native mode the
       pointsize will be set to 10 scaled points.

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

       Normally output characters are constructed from input characters at the
       moment immediately before the character 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 output characters
       in any combination.

       An  output  character  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 will make things clearer.

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

       In GNU mode this will  be  printed  as  \\.   So  each  pair  of  input
       backslashes  ‘\\’  is turned into a single output backslash ‘\’ 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 will be 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.

AUTHORS

       Copyright (C) 1989, 2001, 2002 Free Software Foundation, Inc.

       This document is distributed under the  terms  of  the  FDL  (GNU  Free
       Documentation  License) version 1.1 or later.  You should have received
       a copy of the FDL on your system, it is also available on-line  at  the
       GNU   copyleft   site   〈http://www.gnu.org/copyleft/fdl.html〉.    This
       document was written by  James  Clark,  with  modifications  by  Werner
       Lemberg 〈wl@gnu.org〉 and Bernd Warken 〈bwarken@mayn.de〉.

       This  document  is part of groff, the GNU roff distribution.  Formerly,
       the contents of this document was kept in  the  manual  page  troff(1).
       Only  the parts dealing with the language aspects of the different roff
       systems were carried over into this document.  The troff  command  line
       options and warnings are still documented in troff(1).

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

              shell# man 7 groff

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

       [CSTR #54]
              The Nroff/Troff Users Manual by J. F. Osanna  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〉.