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.\"	Id: eqn.7,v 1.29 2013/07/13 19:41:16 schwarze Exp
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.\" Copyright (c) 2011 Kristaps Dzonsons <kristaps@bsd.lv>
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.Dd July 13, 2013
.Dt EQN 7
.Os
.Sh NAME
.Nm eqn
.Nd eqn language reference for mandoc
.Sh DESCRIPTION
The
.Nm eqn
language is an equation-formatting language.
It is used within
.Xr mdoc 7
and
.Xr man 7
.Ux
manual pages.
It describes the
.Em structure
of an equation, not its mathematical meaning.
This manual describes the
.Nm
language accepted by the
.Xr mandoc 1
utility, which corresponds to the Second Edition eqn specification (see
.Sx SEE ALSO
for references).
.Pp
Equations within
.Xr mdoc 7
or
.Xr man 7
documents are enclosed by the standalone
.Sq \&.EQ
and
.Sq \&.EN
tags.
Equations are multi-line blocks consisting of formulas and control
statements.
.Sh EQUATION STRUCTURE
Each equation is bracketed by
.Sq \&.EQ
and
.Sq \&.EN
strings.
.Em Note :
these are not the same as
.Xr roff 7
macros, and may only be invoked as
.Sq \&.EQ .
.Pp
The equation grammar is as follows, where quoted strings are
case-sensitive literals in the input:
.Bd -literal -offset indent
eqn     : box | eqn box
box     : text
        | \*q{\*q eqn \*q}\*q
        | \*qdefine\*q text text
        | \*qndefine\*q text text
        | \*qtdefine\*q text text
        | \*qgfont\*q text
        | \*qgsize\*q text
        | \*qset\*q text text
        | \*qundef\*q text
        | box pos box
        | box mark
        | \*qmatrix\*q \*q{\*q [col \*q{\*q list \*q}\*q ]*
        | pile \*q{\*q list \*q}\*q
        | font box
        | \*qsize\*q text box
        | \*qleft\*q text eqn [\*qright\*q text]
col     : \*qlcol\*q | \*qrcol\*q | \*qccol\*q | \*qcol\*q
text    : [^space\e\*q]+ | \e\*q.*\e\*q
pile    : \*qlpile\*q | \*qcpile\*q | \*qrpile\*q | \*qpile\*q
pos     : \*qover\*q | \*qsup\*q | \*qsub\*q | \*qto\*q | \*qfrom\*q
mark	: \*qdot\*q | \*qdotdot\*q | \*qhat\*q | \*qtilde\*q | \*qvec\*q
        | \*qdyad\*q | \*qbar\*q | \*qunder\*q
font    : \*qroman\*q | \*qitalic\*q | \*qbold\*q | \*qfat\*q
list    : eqn
        | list \*qabove\*q eqn
space   : [\e^~ \et]
.Ed
.Pp
White-space consists of the space, tab, circumflex, and tilde
characters.
If within a quoted string, these space characters are retained.
Quoted strings are also not scanned for replacement definitions.
.Pp
The following text terms are translated into a rendered glyph, if
available: alpha, beta, chi, delta, epsilon, eta, gamma, iota, kappa,
lambda, mu, nu, omega, omicron, phi, pi, psi, rho, sigma, tau, theta,
upsilon, xi, zeta, DELTA, GAMMA, LAMBDA, OMEGA, PHI, PI, PSI, SIGMA,
THETA, UPSILON, XI, inter (intersection), union (union), prod (product),
int (integral), sum (summation), grad (gradient), del (vector
differential), times (multiply), cdot (centre-dot), nothing (zero-width
space), approx (approximately equals), prime (prime), half (one-half),
partial (partial differential), inf (infinity), >> (much greater), <<
(much less), \-> (left arrow), <\- (right arrow), += (plus-minus), !=
(not equal), == (equivalence), <= (less-than-equal), and >=
(more-than-equal).
.Pp
The following control statements are available:
.Bl -tag -width Ds
.It Cm define
Replace all occurrences of a key with a value.
Its syntax is as follows:
.Pp
.D1 define Ar key cvalc
.Pp
The first character of the value string,
.Ar c ,
is used as the delimiter for the value
.Ar val .
This allows for arbitrary enclosure of terms (not just quotes), such as
.Pp
.D1 define Ar foo 'bar baz'
.D1 define Ar foo cbar bazc
.Pp
It is an error to have an empty
.Ar key
or
.Ar val .
Note that a quoted
.Ar key
causes errors in some
.Nm
implementations and should not be considered portable.
It is not expanded for replacements.
Definitions may refer to other definitions; these are evaluated
recursively when text replacement occurs and not when the definition is
created.
.Pp
Definitions can create arbitrary strings, for example, the following is
a legal construction.
.Bd -literal -offset indent
define foo 'define'
foo bar 'baz'
.Ed
.Pp
Self-referencing definitions will raise an error.
The
.Cm ndefine
statement is a synonym for
.Cm define ,
while
.Cm tdefine
is discarded.
.It Cm gfont
Set the default font of subsequent output.
Its syntax is as follows:
.Pp
.D1 gfont Ar font
.Pp
In mandoc, this value is discarded.
.It Cm gsize
Set the default size of subsequent output.
Its syntax is as follows:
.Pp
.D1 gsize Ar size
.Pp
The
.Ar size
value should be an integer.
.It Cm set
Set an equation mode.
In mandoc, both arguments are thrown away.
Its syntax is as follows:
.Pp
.D1 set Ar key val
.Pp
The
.Ar key
and
.Ar val
are not expanded for replacements.
This statement is a GNU extension.
.It Cm undef
Unset a previously-defined key.
Its syntax is as follows:
.Pp
.D1 define Ar key
.Pp
Once invoked, the definition for
.Ar key
is discarded.
The
.Ar key
is not expanded for replacements.
This statement is a GNU extension.
.El
.Sh COMPATIBILITY
This section documents the compatibility of mandoc
.Nm
and the troff
.Nm
implementation (including GNU troff).
.Pp
.Bl -dash -compact
.It
The text string
.Sq \e\*q
is interpreted as a literal quote in troff.
In mandoc, this is interpreted as a comment.
.It
In troff, The circumflex and tilde white-space symbols map to
fixed-width spaces.
In mandoc, these characters are synonyms for the space character.
.It
The troff implementation of
.Nm
allows for equation alignment with the
.Cm mark
and
.Cm lineup
tokens.
mandoc discards these tokens.
The
.Cm back Ar n ,
.Cm fwd Ar n ,
.Cm up Ar n ,
and
.Cm down Ar n
commands are also ignored.
.El
.Sh SEE ALSO
.Xr mandoc 1 ,
.Xr man 7 ,
.Xr mandoc_char 7 ,
.Xr mdoc 7 ,
.Xr roff 7
.Rs
.%A Brian W. Kernighan
.%A Lorinda L. Cherry
.%T System for Typesetting Mathematics
.%J Communications of the ACM
.%V 18
.%P 151\(en157
.%D March, 1975
.Re
.Rs
.%A Brian W. Kernighan
.%A Lorinda L. Cherry
.%T Typesetting Mathematics, User's Guide
.%D 1976
.Re
.Rs
.%A Brian W. Kernighan
.%A Lorinda L. Cherry
.%T Typesetting Mathematics, User's Guide (Second Edition)
.%D 1978
.Re
.Sh HISTORY
The eqn utility, a preprocessor for troff, was originally written by
Brian W. Kernighan and Lorinda L. Cherry in 1975.
The GNU reimplementation of eqn, part of the GNU troff package, was
released in 1989 by James Clark.
The eqn component of
.Xr mandoc 1
was added in 2011.
.Sh AUTHORS
This
.Nm
reference was written by
.An Kristaps Dzonsons Aq Mt kristaps@bsd.lv .