<s>
Part	O
of	O
the	O
troff	B-Language
suite	O
of	O
Unix	B-Application
document	O
layout	O
tools	O
,	O
eqn	B-Application
is	O
a	O
preprocessor	B-General_Concept
that	O
formats	O
equations	O
for	O
printing	O
.	O
</s>
<s>
A	O
similar	O
program	O
,	O
neqn	O
,	O
accepted	O
the	O
same	O
input	O
as	O
eqn	B-Application
,	O
but	O
produced	O
output	O
tuned	O
to	O
look	O
better	O
in	O
nroff	B-Device
.	O
</s>
<s>
The	O
eqn	B-Application
program	O
was	O
created	O
in	O
1974	O
by	O
Brian	O
Kernighan	O
and	O
Lorinda	O
Cherry	O
.	O
</s>
<s>
It	O
was	O
implemented	O
using	O
yacc	B-Application
compiler-compiler	O
.	O
</s>
<s>
The	O
input	O
language	O
used	O
by	O
eqn	B-Application
allows	O
the	O
user	O
to	O
write	O
mathematical	O
expressions	O
in	O
much	O
the	O
same	O
way	O
as	O
they	O
would	O
be	O
spoken	O
aloud	O
.	O
</s>
<s>
The	O
eqn	B-Application
language	O
is	O
similar	O
to	O
the	O
mathematical	O
component	O
of	O
TeX	B-Application
,	O
which	O
appeared	O
several	O
years	O
later	O
,	O
but	O
is	O
simpler	O
and	O
less	O
complete	O
.	O
</s>
<s>
An	O
independent	O
compatible	O
implementation	O
of	O
the	O
eqn	B-Application
preprocessor	B-General_Concept
has	O
been	O
developed	O
by	O
GNU	B-Operating_System
as	O
part	O
of	O
groff	B-Application
,	O
the	O
GNU	B-Operating_System
version	O
of	O
troff	B-Language
.	O
</s>
<s>
The	O
GNU	B-Operating_System
implementation	O
extends	O
the	O
original	O
language	O
by	O
adding	O
a	O
number	O
of	O
new	O
keywords	O
such	O
as	O
smallover	O
and	O
accent	O
.	O
</s>
<s>
mandoc	B-Application
,	O
a	O
specialised	O
compiler	O
for	O
UNIX	B-Application
man	B-Protocol
pages	I-Protocol
,	O
also	O
contains	O
a	O
standalone	O
eqn	B-Application
parser/formatter	O
.	O
</s>
<s>
Eqn	B-Application
was	O
done	O
by	O
using	O
yacc	B-Application
parser	O
generator	O
.	O
</s>
<s>
Here	O
is	O
how	O
some	O
examples	O
would	O
be	O
written	O
in	O
eqn	B-Application
(	O
with	O
equivalents	O
in	O
TeX	B-Application
for	O
comparison	O
)	O
:	O
</s>
<s>
Spaces	O
are	O
important	O
in	O
eqn	B-Application
;	O
tokens	O
are	O
delimited	O
only	O
by	O
whitespace	O
characters	O
,	O
tildes	O
~	O
,	O
braces	O
 {  } 	O
and	O
double-quotes	O
""	O
.	O
</s>
