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+.\"
+.\" Permission to use, copy, modify, and distribute this software for any
+.\" purpose with or without fee is hereby granted, provided that the above
+.\" copyright notice and this permission notice appear in all copies.
+.\"
+.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+.\"
+.\"
+.\" Copyright (c) 2011 Kristaps Dzonsons <kristaps@bsd.lv>
+.\" Copyright 2012 Nexenta Systems, Inc. All rights reserved.
+.\"
+.Dd Jul 19, 2014
+.Dt EQN 5
+.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 5
+and
+.Xr man 5
+.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 5
+or
+.Xr man 5
+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 5
+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
+.Xr mandoc 1 ,
+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
+.Xr mandoc 1 ,
+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
+.Xr mandoc 1
+.Nm
+and the
+.Xr troff 1
+.Nm
+implementation (including GNU troff).
+.Pp
+.Bl -dash -compact
+.It
+The text string
+.Sq \e\*q
+is interpreted as a literal quote in
+.Xr troff 1 .
+In
+.Xr mandoc 1 ,
+this is interpreted as a comment.
+.It
+In
+.Xr troff 1 ,
+The circumflex and tilde white-space symbols map to
+fixed-width spaces.
+In
+.Xr mandoc 1 ,
+these characters are synonyms for the space character.
+.It
+The
+.Xr troff 1 ,
+implementation of
+.Nm
+allows for equation alignment with the
+.Cm mark
+and
+.Cm lineup
+tokens.
+.Xr mandoc 1
+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 5 ,
+.Xr mandoc_char 5 ,
+.Xr mdoc 5 ,
+.Xr roff 5
+.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 ,
+.Mt kristaps@bsd.lv .
