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.Dd $Mdocdate$
.Dt EQN 7
.Os
.Sh NAME
.Nm eqn
.Nd eqn language reference for mandoc
.Sh DESCRIPTION
The
.Nm eqn
language is a equation-formatting language.
It is used within
.Xr mdoc 7
and
.Xr man 7
.Ux
manual pages.
This manual describes the subset of the
.Nm
language accepted by the
.Xr mandoc 1
utility.
.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:
.Bd -literal -offset indent
eqn     : box | eqn box
box     : text
        | { eqn }
        | DEFINE text text
        | SET text text
        | UNDEF text
        | box pos box
        | box mark
        | font box
        | SIZE text box
text    : TEXT
pos     : OVER
        | SUP
        | SUB
        | TO
        | FROM
mark	: DOT
        | DOTDOT
        | HAT
        | TILDE
        | VEC
        | DYAD
        | BAR
        | UNDER
font    : ROMAN
        | ITALIC
        | BOLD
.Ed
.Pp
Data in TEXT form is a non-empty sequence of non-space characters or a
non-empty quoted string.
Unless within a quoted string, white-space (and enclosing literal quote
pairs) is thrown away.
Quoted strings are not scanned for replacement definitions.
.Pp
The following control statements are available:
.Bl -tag -width Ds
.It Cm define
Replace all occurances 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.
.It Cm set
Set an equation mode.
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.
.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.
.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.
.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 kristaps@bsd.lv .