GROFF_DIFF(7)                                                    GROFF_DIFF(7)



NAME
       groff_diff - differences between GNU troff and classical troff

DESCRIPTION
       This  manual page describes the language differences between groff, the
       GNU roff text processing system and the classical roff formatter of the
       freely  available  Unix  7 of the 1970s, documented in the Troff User's
       Manual by Osanna and Kernighan.  This inludes the roff language as well
       as the intermediate output format (troff output).

       The  section SEE ALSO gives pointers to both the classical roff and the
       modern groff documentation.

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

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

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

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

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

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

       \n[xxx]
              Interpolate number register xxx.

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

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

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

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

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

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

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

       e1>?e2 The maximum of e1 and e2.

       e1<?e2 The minimum of e1 and e2.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

              \O4    End a nesting level.

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

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

                     .nr name +-n

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

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

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

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

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

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

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

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

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

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

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

              will print 4.

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

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

       \)     Like  \&  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.

       .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.

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

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

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

       .asciify xx
              This  request  `unformats'  the  diversion xx in such a way that
              ASCII and space characters (and some escape sequences) that were
              formatted  and  diverted  into  xx 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.

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

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

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

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

       .brp   This is the same as \p.

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

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

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

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

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

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

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

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

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

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

              A glyph definition can be removed with the rchar request.

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

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

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

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

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

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

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

                     .defcolor darkgreen rgb 0.1f 0.5f 0.2f

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

                     .defcolor darkgreen rgb 0.1 0.5 0.2

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

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

       .dei xx yy
              Define macro indirectly.  The following example

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

              is equivalent to

                     .de aa bb

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

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

                     .do fam T

              would have the same effect as

                     .fam T

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

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

       .ecs   Save current escape character.

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

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

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

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

       .fcolor c
              Set  the fill color to c.  If c is missing, switch to the previ-
              ous fill color.

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

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

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

       .gcolor c
              Set the glyph color to c.  If c is missing, switch to the previ-
              ous glyph color.

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

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

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

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

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

              o      No support for `digraphs' like \$.

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

              o      No macro expansion.

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

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

              o      \endinput is recognized also.

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

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

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

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

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

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

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

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

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

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

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

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

              yields

                     a         b         c

              In line-tabs mode, the same code gives

                     a         b                   c

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       .rnn xx yy
              Rename number register xx to yy.

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

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

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

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

       .special s1 s2...
              Fonts s1, s2, are special and will be searched for glyphs not in
              the current font.  Without arguments, reset the list of  special
              fonts to be empty.

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

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

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

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

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

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

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

                     .di x
                     .trf f
                     .di

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

       .trin abcd
              This is the same as the  tr  request  except  that  the  asciify
              request  will use the character code (if any) before the charac-
              ter translation.  Example:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

                     .ta T .5i

              will set tabs every half an inch.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       Other available read/write number registers are:

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

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

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

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

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

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

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

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

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

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

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

              .xx \\$@

       is

              \\*[xx]\\

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

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

       There are some new types of condition:

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

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

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

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

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

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

       The tr request can now map characters onto \~.

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

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

INTERMEDIATE OUTPUT FORMAT
       This section describes the format output by GNU troff.  The output for-
       mat used by GNU troff is very similar to that used by Unix device-inde-
       pendent troff.  Only the differences are documented here.

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

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

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

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

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

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

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

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

       For color support, some new commands have been added:

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

       The x device control command has been extended.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       Another set of extensions is

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

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

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

       The first three output commands are guaranteed to be:

              x T device
              x res n h v
              x init

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

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

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

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

              .dsabcd

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

AUTHORS
       Copyright (C) 1989, 2001, 2002, 2003, 2004  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
              escapes  of  plain groff.  From the command line, this is called
              using

              shell# man 7 groff

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

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



Groff Version 1.19.2            6 February 2006                  GROFF_DIFF(7)
