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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
       request,  the second and fourth arguments to the tkf request, the argu-
       ment to the \H escape sequence, and those variants  of  the  \s  escape
       sequence that take a numeric expression as their argument.

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

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

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

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

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

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

       e1>>?e2 The maximum of e1 and e2.

       e1<&lt;?e2 The minimum of e1 and e2.

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

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

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

       \C'xxx'
              Typeset  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
              superscripting could be defined like this

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

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

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

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

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

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

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

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

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

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

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

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

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

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

              \O4    End a nesting level.

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

       \R'name +-n'
              This has the same effect as

                     .nr name +-n

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

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

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

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

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

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

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

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

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

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

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

              will print 4.

       \/     This increases the width of the preceding 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 \&&amp; except that it behaves like a  character  declared  with
              the cflags request to be transparent for the purposes of end-of-
              sentence recognition.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       .brp   This is the same as \p.

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

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

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

              4      Lines  can be broken after the character (initially char-
                     acters -\(hy\(em have this property); a line will not  be
                     broken at a character with this property unless the 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
              escape  sequences; words containing the character can be hyphen-
              ated correctly, if the hcode request is used to give the charac-
              ter 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
              acceptable argument to the write request.  See the open request.

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

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

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

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

                     .defcolor darkgreen rgb 0.1f 0.5f 0.2f

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

                     .defcolor darkgreen rgb 0.1 0.5 0.2

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

       .dei xx yy
              Define macro indirectly.  The following example

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

              is equivalent to

                     .de aa bb

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

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

                     .do fam T

              would have the same effect as

                     .fam T

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

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

       .ecs   Save current escape character.

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

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

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

       .fchar c string
              Define fallback 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
              request.

       .hla lang
              Set  the  current  hyphenation  language  to  lang.  Hyphenation
              exceptions specified with the hw request  and  hyphenation  pat-
              terns  specified  with  the hpf request are both associated with
              the current hyphenation language.  The hla  request  is  usually
              invoked by the troffrc file.

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

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

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

              o      No support for `digraphs' like \$.

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

              o      No macro expansion.

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

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

              o      \endinput is recognized also.

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

              Use the hpfcode request to map the encoding used in  hyphenation
              patterns files to groff's input encoding.

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

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

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

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

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

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

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

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

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

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

              yields

                     a         b         c

              In line-tabs mode, the same code gives

                     a         b                   c

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

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

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

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

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

       .opena stream filename
              Like open, but if filename exists, append to it instead of trun-
              cating it.

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

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

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

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

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

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

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

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

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

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

       .rnn xx yy
              Rename number register xx to yy.

       .shc c Set  the  soft hyphen character to c.  If c is omitted, the soft
              hyphen character will be set to  the  default  \(hy.   The  soft
              hyphen  character is the character which will be inserted when a
              word is hyphenated at a line break.  If the soft hyphen  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
              request) nor translations (specified with the  tr  request)  are
              considered when finding the soft hyphen character.

       .shift n
              In  a  macro,  shift  the  arguments  by n positions: argument i
              becomes argument i-n; arguments 1 to n will no longer be  avail-
              able.   If n is missing, arguments will be shifted by 1.  Shift-
              ing by negative amounts is currently undefined.

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

       .special s1 s2...
              Fonts  s1,  s2,  are special and will be searched for 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
              index 0.  If n2 is omitted, it is  taken  to  be  equal  to  the
              string's  length.   If  the index value n1 or n2 is negative, it
              will be counted from the end of the string, going backwards: The
              last character has index -1, the character before the last char-
              acter has index -2, etc.

       .tkf f s1 n1 s2 n2
              Enable track kerning for font f.  When the current font is f the
              width  of every 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
              request will use the character code (if any) before the  charac-
              ter translation.  Example:

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

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

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

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

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

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

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

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

       .warn n
              Control warnings.  n is the sum of the numbers  associated  with
              each  warning  that is to be enabled; all other warnings will be
              disabled.  The number associated with each warning is listed  in
              troff(1).   For  example, .warn 0 will disable all warnings, and
              .warn 1 will disable all  warnings  except  that  about  missing
              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
              request  in  nroff mode also; a given value is then rounded down
              to the nearest multiple of 12.  The sentence space size is  used
              in  two  circumstances.   If the end of a sentence occurs at the
              end of a line in fill mode, then both an inter-word space and  a
              sentence  space will be added; if two spaces follow the end of a
              sentence in the middle of a line, then the second space will  be
              a sentence space.  Note that the behaviour of UNIX troff will be
              exactly that exhibited by GNU troff  if  a  second  argument  is
              never  given to the ss request.  In GNU troff, as in UNIX troff,
              you should always follow a sentence with either a newline or two
              spaces.

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

                     .ta T .5i

              will set tabs every half an inch.

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

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

       \n[.cdp]
              The  depth  of  the last character added to the current 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
              request, or the number of the current page plus 1.

       \n[.ps]
              The current pointsize in scaled points.

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

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

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

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

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

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

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

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

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

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

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

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

       \n[llx]
       \n[lly]
       \n[urx]
       \n[ury]
              These four registers are set by the .psbb  request  and  contain
              the  bounding  box values (in PostScript units) of a given 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 \&&amp; between them.

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

       There are some new types of condition:

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

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

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

       .if cch
              True if there is a 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,
              appropriately  scaled for the current point size, and rounded so
              that it is a multiple of  the  horizontal  resolution.   Special
              characters cannot be printed using this command.

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

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

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

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

       For color support, some new commands have been added:

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

       The x device control command has been extended.

       x u n  If n is 1, start underlining of spaces.  If n is 0, stop  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
              position.  delim $$

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

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

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

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

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

       where c is not one of c, e, l, a, or ~, Unix troff will treat  each  of
       the  $x sub i$ as a horizontal quantity, and each of the $y sub i$ as a
       vertical quantity and will assume that the width of the drawn object is
       $sum  from i=1 to n x sub i$, and that the height is $sum from i=1 to n
       y sub i$.  (The assumption about the height can be  seen  by  examining
       the  st  and  sb  registers after using such a D command in a \w escape
       sequence).  This rule also holds for all the original drawing  commands
       with the exception of De.  For the sake of compatibility GNU troff also
       follows this rule, even though it produces an ugly result in  the  case
       of  the Dt, and, 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),
       \&&amp; (ampersand), \{ (opening brace), \} (closing brace),  '\ '  (space),
       \'  (single  quote),  \`  (backquote),  \-  (minus), \_ (underline), \!
       (bang), \% (percent), and \c (character c) in names of strings, macros,
       diversions,  number registers, fonts or environments, whereas classical
       troff does.

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

       Fractional pointsizes cause one noteworthy incompatibility.  In classi-
       cal troff, the ps request ignores scale indicators and so

              .ps 10u

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

       In  groff  mode,  there is a fundamental difference between unformatted
       input characters, and formatted  output  characters.   Everything  that
       affects how an output character will be output is stored with the char-
       acter; 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
       resulting  output  backslashes are not interpreted as escape characters
       when they are reread.

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

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

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

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

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

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

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

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

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

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

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

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

              shell# man 7 groff

       roff(7)
              A survey of roff systems, including pointers to further histori-
              cal documentation.

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



Groff Version 1.18.1               Nov  2002                     GROFF_DIFF(7)