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GROFF_DIFF(7)          Miscellaneous Information Manual          GROFF_DIFF(7)



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 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.

GROFF LANGUAGE
       In this section, all additional features of groff compared to the clas-
       sical Unix 7 troff are described in detail.

   Long names
       The  names  of number registers, fonts, strings/macros/diversions, spe-
       cial characters (glyphs), 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.

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

       \[comp1 comp2 ...]
              Print composite glyph consisting of multiple components.   Exam-
              ple:  `\[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 list of glyph name components used compos-
              ite glyph names.

       \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 spec-
       ified  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 argu-
       ment 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  scal-
       ing  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<&lt;?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 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.

       \B'anything'
              This  expands  to 1 or 0 resp., depending on whether anything is
              or is not a valid numeric expression.  It will return 0 if  any-
              thing 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 inter-
              preted 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.

       \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 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  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]  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  speci-
                     fied  and must be one of l, r, c, or i (left, right, cen-
                     tered, inline).  filename will  be  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 exten-
              sion to the UNIX troff output format, and will  confuse  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.

       \$(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  glyph  so  that  the
              spacing  between that glyph and the following glyph will be cor-
              rect if the following glyph is a roman glyph.  It is a good idea
              to  use this escape sequence whenever an italic glyph is immedi-
              ately 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 will 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 \&&amp; 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 `compati-
              bility 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 compatibil-
              ity 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 char-
                     acters -\[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]\[radicalex]\[sqrtex] 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 hav-
                     ing a zero space  factor  in  TeX  (initially  characters
                     "')]*\(dg\(rq have this property).

       .char c string
              Define  glyph  c  to  be string.  Every time glyph c needs to be
              printed, string will be processed in a temporary environment and
              the result will be wrapped up into a single object.  Compatibil-
              ity 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 glyphs in string.

              A  glyph  defined by this request can be used just like a normal
              glyph provided by the output device.  In particular other  char-
              acters  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 cor-
              rectly,  if  the  hcode  request is used to give the character a
              hyphenation code.

              There is a special anti-recursion feature: Use of  glyph  within
              the  glyph's  definition  will be 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.

       .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,  other-
              wise  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  compo-
              nents),  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 spec-
              ify 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 fam-
              ilies.

       .fchar c string
              Define fallback 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.

       .fschar f c string
              Define  fallback 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,... will be special,
              that is, they will searched for glyphs not in the current  font.
              Any  fonts  specified  in  the  special request will be searched
              after fonts specified in the fspecial  request.   Without  argu-
              ment, 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, or in the ft, ul, bd,  cs,  tkf,  spe-
              cial,  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.   Ini-
              tially  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 pat-
              terns 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 asso-
              ciated 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.

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

              o      No support for `digraphs' like \$.

              o      ^^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.

              o      No macro expansion.

              o      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.

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

              o      \endinput is recognized also.

              o      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  com-
                     ment).

              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 pat-
              terns 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  lan-
              guage.

       .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  hyphen-
              ation  margin  is  associated with the current environment.  The
              current hyphenation margin is available in the  \n[.hym]  regis-
              ter.

       .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 regis-
              ter 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 dis-
              tances are computed relative to the (current) output line.  Oth-
              erwise  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 trun-
              cating 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 bound-
              ing 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 glyphs c1,  c2,...   This  undoes  the
              effect of a char request.

       .return
              Within a macro, return immediately.  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 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 spe-
              cial request but before the mounted special fonts.

       .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 glyph 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 glyph 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 avail-
              able.  If n is missing, arguments will be shifted by 1.   Shift-
              ing 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 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 will
              cause a warning if troff must add 0.2m or more for  each  inter-
              word  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 exam-
              ple, 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 fspe-
              cial 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 char-
              acter 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 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 writ-
              ten 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 sub-
              ject 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  charac-
              ter 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 condi-
              tion 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,  .unfor-
              mat  will  only  handle  tabs  and spaces between words (usually
              caused by spaces or newlines in the input) specially.  The  for-
              mer 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 posi-
              tion traps.  The parameter that controls whether vertical  posi-
              tion  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
              glyphs.  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 previ-
              ously 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 escape sequences
       \D'...'
              All   drawing   commands  of  groff's  intermediate  output  are
              accepted.  See subsection Drawing Commands below for more infor-
              mation.

   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  environ-
              ments.   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 argu-
              ment  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 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 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[.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 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[.slant]
              The slant of the current font as set with \S.

       \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  Post-
              Script 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 argu-
              ment, 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[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 regis-
              ter \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,
       glyphs  from  that font will be kerned.  Kerning between two glyphs can
       be inhibited by placing a \&&amp; between them.

       In a string comparison in a condition, characters that appear  at  dif-
       ferent input levels to the first delimiter character will not be recog-
       nised as the second or third delimiters.  This applies also to  the  tl
       request.   In  a \w escape sequence, a character that appears at a dif-
       ferent 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 imple-
       mentation 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 rec-
       ognized  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 glyph ch available; ch  is  either  an  ASCII
              character  or  a  glyph  (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, I'm `foo'.
              . nop I will now define `bar'.
              . de bar
              . nop Hello, I'm `bar'.
              . .
              . nop Done.
              ..
              .foo
              .bar

INTERMEDIATE OUTPUT FORMAT
       This section describes the format output by GNU troff.  The output for-
       mat used by GNU troff is very similar to that used by Unix device-inde-
       pendent 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 will use 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 char-
              acter  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 will be only two characters long.

       When a glyph is to be printed, that glyph will always be 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  under-
              lining  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 cor-
              respond 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.

              The  corresponding \D'f...'  command shouldn't be used since its
              argument is always rounded to an integer multiple of  the  hori-
              zontal 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 ver-
              tical  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.  Tradition-
              ally 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 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  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 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 com-
       mand 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 consis-
       tent 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),
       \&&amp; (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 classi-
       cal 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 (glyphs).  Everything
       that  affects how a glyph will be 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 combi-
       nation.

       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 will make things clearer.

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

       In GNU mode this will be printed as \\.  So each pair  of  input  back-
       slashes '\\' 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  charac-
       ter  is  not  redefined.   It  works in both GNU mode and compatibility
       mode, while dirty tricks like specifying a sequence of  multiple  back-
       slashes 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:

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

       o The  intermediate output cannot be rescaled to other devices as clas-
         sical "device-independent" troff did.

AUTHORS
       Copyright (C) 1989, 2001, 2002, 2003 Free Software Foundation, Inc.

       This document is distributed under the terms of the FDL (GNU Free Docu-
       mentation  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
       <wlATgnu.org> and Bernd Warken <bwarkenATmayn.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 com-
              plete  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 histori-
              cal documentation.

       [CSTR #54]
              The Nroff/Troff User's 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>;.



Groff Version 1.19               30 June 2003                    GROFF_DIFF(7)