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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.

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

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

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

       \[xxx]	 Print the special character called xxx.

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

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

       \n[xxx]	 Interpolate number register xxx.

   Fractional pointsizes
       A scaled	point is equal to 1/sizescale points, where sizescale is 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 re-
       quest, the second and fourth arguments to the tkf request, the argument
       to the \H escape	sequence, and those variants of	the \s escape sequence
       that take a numeric expression as their argument.

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

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

       It would	make no	sense to use the z scale indicator in  a  numeric  ex-
       pression	 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  re-
       quest.  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 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  character named xxx.  Normally it is more convenient to
	      use \[xxx].  But \C has the advantage that it is compatible with
	      recent versions of UNIX and is available in compatibility	mode.

       \E     This is equivalent to an escape character, but it	is not	inter-
	      preted  in copy-mode.  For example, strings to start and end su-
	      perscripting 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 character with code n	in the current font.  n	can be
	      any integer.  Most devices only have characters with  codes  be-
	      tween 0 and 255.	If the current font does not contain a charac-
	      ter  with	that code, special fonts will not be searched.	The \N
	      escape sequence can be conveniently used in conjunction with the
	      char request, for	example

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

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

       \On
       \O[n]  Suppressing  troff  output.   The	escapes	\02, \O3, \O4, and \O5
	      are intended for internal	use by grohtml.

	      \O0    Disable any ditroff glyphs	from being emitted to the  de-
		     vice 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[op-
		     miny], \n[opmaxx],	and \n[opmaxy] to -1.  These four reg-
		     isters 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, en-
		     able 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  es-
		     cape occurs at the	outer nesting level, write filename to
		     stderr.   The position of the image, P, must be specified
		     and must be one of	l, r, c, or i (left, right,  centered,
		     inline).  filename	will be	associated with	the production
		     of	the next inline	image.

       \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 re-
	      turned 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 po-
	      sition; anything may not contain tabs or leaders.

       \$0    The name by which	the current macro was invoked.	 The  als  re-
	      quest 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  di-
	      version  is  reread, anything will be interpreted.  anything may
	      not contain newlines; use	\! if you want to embed	newlines in  a
	      diversion.   The	escape	sequence \? is also recognised in copy
	      mode and turned into a single internal code;  it	is  this  code
	      that terminates anything.	 Thus

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

	      will print 4.

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

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

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

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

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

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

   New requests
       .aln xx yy
	      Create an	alias xx for number register object named yy.  The new
	      name and the old name will be exactly equivalent.	 If yy is  un-
	      defined,	a  warning  of type reg	will be	generated, and the re-
	      quest will be ignored.

       .als xx yy
	      Create an	alias xx for request, string, macro, or	diversion  ob-
	      ject  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 re-
	      quests only create a new object if the name of the macro,	diver-
	      sion  or string diversion	is currently undefined or if it	is de-
	      fined to be a request; normally they modify the value of an  ex-
	      isting 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  re-
	      quests  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 in-
	      put characters when xx is	reread.	 Useful	for diversions in con-
	      junction 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 re-
	      quests.  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 char-
		     acters on each side both have non-zero hyphenation	codes.

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

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

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

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

	      A	character defined by this request can be used just like	a nor-
	      mal  character  provided	by  the	 output	device.	 In particular
	      other characters can be translated to it with the	tr request; it
	      can be made the leader character by  the	lc  request;  repeated
	      patterns can be drawn with the character using the \l and	\L es-
	      cape sequences; words containing the character can be hyphenated
	      correctly,  if the hcode request is used to give the character a
	      hyphenation code.

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

	      A	character definition can be removed with the rchar request.

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

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

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

       .color n
	      If n is non-zero or missing, enable  colors  (this  is  the  de-
	      fault), 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  (ma-
	      genta).	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 re-
	      defined.	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	character  c to	be string.  The	syntax of this
	      request is the same as the char request; the only	difference  is
	      that a character defined with char hides the glyph with the same
	      name in the current font,	whereas	a character defined with fchar
	      is  checked only if the particular glyph isn't found in the cur-
	      rent font.  This test happens before checking special fonts.

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

       .ftr f g
	      Translate	 font  f to g.	Whenever a font	named f	is referred to
	      in an \f escape sequence,	or in the ft, ul, bd,  cs,  tkf,  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 re-
	      quest.

       .hla lang
	      Set  the	current	hyphenation language to	lang.  Hyphenation ex-
	      ceptions specified with the hw request and hyphenation  patterns
	      specified	with the hpf request are both associated with the cur-
	      rent  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.

	      	     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  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 in-
	      voked  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 re-
	      cently read patterns to character	code b,	character code c to d,
	      etc.  Initially, all character codes map to themselves.  The ar-
	      guments of hpfcode must be integers in the range 0 to 255.  Note
	      that  it	is  even possible to use character codes which are in-
	      valid 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  de-
	      fault 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 ap-
	      plied after the line is output.

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

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

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

       .rnn xx yy
	      Rename number register xx	to yy.

       .shc c Set  the	soft hyphen character to c.  If	c is omitted, the soft
	      hyphen character will be set to the default \(hy.	 The soft  hy-
	      phen  character  is  the character which will be inserted	when a
	      word is hyphenated at a line break.  If the soft hyphen  charac-
	      ter does not exist in the	font of	the character immediately pre-
	      ceding a potential break point, then the line will not be	broken
	      at that point.  Neither definitions (specified with the char re-
	      quest) nor translations (specified with the tr request) are con-
	      sidered when finding the soft hyphen character.

       .shift n
	      In  a  macro, shift the arguments	by n positions:	argument i be-
	      comes 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  characters
	      not in the current font.

       .spreadwarn limit
	      Make  troff  emit	a warning if the additional space inserted for
	      each space between words in an output line is larger or equal to
	      limit.  A	negative value is changed to zero; no argument toggles
	      the warning on and off  without  changing	 limit.	  The  default
	      scaling  indicator is m.	At startup, spreadwarn is deactivated,
	      and limit	is set to  3m.	 For  example,	.spreadwarn 0.2m  will
	      cause  a	warning	if troff must add 0.2m or more for each	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  in-
	      dex  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 character	will be	increased by an	amount between
	      n1 and n2; when the current point	size is	less than or equal  to
	      s1 the width will	be increased by	n1; when it is greater than or
	      equal  to	 s2  the width will be increased by n2;	when the point
	      size is greater than or equal to s1 and less than	or equal to s2
	      the increase in width is a linear	function of the	point size.

       .tm1 string
	      Similar to the tm	request, string	is read	in copy	mode and 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  re-
	      quest  will use the character code (if any) before the character
	      translation.  Example:

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

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

       .trnt abcd
	      This is the same as the tr request except	that the  translations
	      do not apply to text that	is transparently throughput into a di-
	      version 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
	      characters.  If n	is not given, all warnings will	be enabled.

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

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

       .write stream anything
	      Write anything to	the stream named stream.  stream  must	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 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 re-
	      quest  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 sen-
	      tence space will be added; if two	spaces follow  the  end	 of  a
	      sentence	in the middle of a line, then the second space will be
	      a	sentence space.	 Note that the behaviour of UNIX troff will be
	      exactly that exhibited by	GNU troff  if  a  second  argument  is
	      never  given to the ss request.  In GNU troff, as	in UNIX	troff,
	      you should always	follow a sentence with either a	newline	or two
	      spaces.

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

		     .ta T .5i

	      will set tabs every half an inch.

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

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

       \n[.cdp]
	      The  depth  of  the last character added to the current environ-
	      ment.  It	is positive if the character extends below  the	 base-
	      line.

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

       \n[.cht]
	      The height of the	last character added to	the  current  environ-
	      ment.   It  is positive if the character extends above the base-
	      line.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       \n[.pn]
	      The number of the	next page, either the value set	by  a  pn  re-
	      quest, 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 re-
	      quest.

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

       \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 en-
	      abled  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 characters.

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

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

       Other available read/write number registers are:

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

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

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

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

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

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

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

       \n[year]
	      The current year.	 Note that the traditional troff number	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,
       characters from that font will be kerned.  Kerning between two  charac-
       ters can	be inhibited by	placing	a \& 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 character ch available; ch is either an ASCII
	      character	 or  a special character \(xx or \[xxx]; the condition
	      will also	be true	if ch has been defined by the char request.

       The tr request can now map characters onto \~.

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

	      .de foo
	      .	nop Hello, 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  character	with  index  n (a non-negative integer)	of the
	      current font.

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

       txxx   xxx  is  any  sequence  of characters terminated by a space or a
	      newline; the first character should be printed  at  the  current
	      position,	the current horizontal position	should be increased by
	      the  width of the	first character, and so	on for each character.
	      The width	of the character is that given in the font  file,  ap-
	      propriately  scaled  for	the current point size,	and rounded so
	      that it is a multiple of	the  horizontal	 resolution.   Special
	      characters cannot	be printed using this command.

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

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

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

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

       For color support, some new commands have been added:

       mc cyan magenta yellow
       md
       mg gray
       mk cyan magenta yellow black
       mr red green blue
	      Set  the	color  components  of the current drawing color, using
	      various color schemes.  md resets	the drawing color to  the  de-
	      fault  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.

       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 po-
	      sition.  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 se-
       quence).	 This rule also	holds for all the  original  drawing  commands
       with the	exception of De.  For the sake of compatibility	GNU troff also
       follows	this  rule, even though	it produces an ugly result in the case
       of the Dt, and, to a lesser extent, DE commands.	 Thus after  executing
       a D command of the form

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

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

       Another set of extensions is

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

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

   Device Control Commands
       There  is  a  continuation convention which permits the argument	to the
       x X command to contain newlines:	when outputting	the  argument  to  the
       x X  command, GNU troff will follow each	newline	in the argument	with a
       + character (as usual, it will terminate	the  entire  argument  with  a
       newline);  thus	if  the	line after the line containing the x X command
       starts with +, then the newline ending the line containing the x	X 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),
       \&  (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  se-
       quences 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.  Everything that af-
       fects how an output character will be output is stored with the charac-
       ter;  once an output character has been constructed it is unaffected by
       any subsequent requests that are	executed, including the	bd,  cs,  tkf,
       tr, or fp requests.

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

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

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

       In  GNU	mode  this will	be printed as \\.  So each pair	of input back-
       slashes `\\' is turned into a single output backslash `\' and  the  re-
       sulting	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:

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

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

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

       This document is	distributed under the terms of the FDL (GNU Free 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
       <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 com-
	      plete  reference	of all predefined requests, registers, and es-
	      capes of plain groff.  From the command line, this is called us-
	      ing

	      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 re-
	      vision of	Brian Kernighan	of 1992, being the classical troff
	      documentation <http://cm.bell-labs.com/cm/cs/cstr/54.ps.gz>.

Groff Version 1.18.1		 12 April 2025			 GROFF_DIFF(7)

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