Provided by: groff_1.21-7_amd64 bug

NAME

       groff_diff - differences between GNU troff and classical troff

DESCRIPTION

       This  manual  page  describes  the  language  differences between groff, the GNU roff text
       processing system, and the classical roff formatter of the freely available Unix 7 of  the
       1970s,  documented  in the Troff User's Manual by Ossanna and Kernighan.  This 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 classical Unix 7 troff
       are described in detail.

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

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

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

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

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

       \n[xxx]
              Interpolate number register xxx.

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

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

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

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

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

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

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

       e1>?e2 The maximum of e1 and e2.

       e1<?e2 The minimum of e1 and e2.

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

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

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

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

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

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

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

       \Ff
       \F(fm
       \F[fam]
              Change  font  family.   This is the same as the fam request.  \F[] switches back to
              the previous 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 are not searched.  The \N escape
              sequence can be conveniently used in conjunction with the char request, for example

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

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

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

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

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

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

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

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

              \O4    End a nesting level.

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

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

                     .nr name ±n

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

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

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

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

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

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

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

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

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

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

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

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

              prints 4.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       .brp   This is the same as \p.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

              A glyph definition can be removed with the rchar request.

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

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

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

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

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

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

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

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

                     .defcolor darkgreen rgb 0.1f 0.5f 0.2f

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

                     .defcolor darkgreen rgb 0.1 0.5 0.2

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

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

       .dei xx yy
              Define macro indirectly.  The following example

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

              is equivalent to

                     .de aa bb

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

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

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

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

                     .do fam T

              would have the same effect as

                     .fam T

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

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

       .ecs   Save current escape character.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

              ·      No support for `digraphs' like \$.

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

              ·      No macro expansion.

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

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

              ·      \endinput is recognized also.

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

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

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

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

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

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

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

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

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

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

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

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

              yields

                     a         b         c

              In line-tabs mode, the same code gives

                     a         b                   c

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

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

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

       .nop anything
              Execute anything.  This is similar to `.if 1'.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       .rnn xx yy
              Rename number register xx to yy.

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

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

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

       .sizes s1 s2 ... sn [0]
              This command is similar to the sizes command of a DESC file.  It sets the available
              font sizes for the current font to s1, s2, ..., sn  scaled  points.   The  list  of
              sizes  can be terminated by an optional 0.  Each si can also be a range of sizes 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 are searched for glyphs not in the current font.
              Without arguments, reset the list of special fonts to be empty.

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

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

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

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

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

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

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

                     .di x
                     .trf f
                     .di

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

                     .ta T .5i

              sets tabs every half an inch.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       Other available read/write number registers are:

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

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

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

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

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

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

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

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

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

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

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

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

              .xx \\$@

       is

              \\*[xx]\\

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

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

       There are some new types of condition:

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

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

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

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

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

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

       The tr request can now map characters onto \~.

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

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

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

INTERMEDIATE OUTPUT FORMAT

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

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

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

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

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

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

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

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

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

       For color support, some new commands have been added:

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

       The x device control command has been extended.

       x u n  If n is 1, start underlining of spaces.  If n is 0,  stop  underlining  of  spaces.
              This is needed for the cu request in nroff mode and is ignored otherwise.

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

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

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

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

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

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

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

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

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

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

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

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

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

       Another set of extensions is

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

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

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

       The first three output commands are guaranteed to be:

              x T device
              x res n h v
              x init

INCOMPATIBILITIES

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

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

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

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

              .dsabcd

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

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

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

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

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

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

              .ps 10u

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

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

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

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

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

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

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

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

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

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

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

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

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

AUTHORS

       Copyright  (C)  1989,  2001,  2002, 2003, 2004, 2006, 2007, 2008, 2009, 2010 Free Software
       Foundation, Inc.

       This document is distributed under the terms of the FDL (GNU Free  Documentation  License)
       version  1.3  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

                     man 7 groff

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

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