` ` ` ` ` `To tell
EQN
where a mathematical expression begins and ends,
we mark it with lines beginning
.EQ
and
.EN.` `
Thus
if you type the lines
^EQ

x=y+z

^EN

your output will look like

The
.EQ
and
.EN
are copied through untouched;
they
are not otherwise processed
by
EQN.` `
This means that you have to take care
of things like centering, numbering, and so on
yourself.` `
The most common way is to use the
TROFF
and
NROFF
macro package package `-ms'
developed by M. E. Lesk[3],
which allows you to center, indent, left-justify and number equations.` `

` ` ` ` ` `With the `-ms' package,
equations are centered by default.` `
To left-justify an equation, use
.EQ L
instead of
.EQ.` `
To indent it, use
.EQ I.` `
Any of these can be followed by an arbitrary `equation number'
which will be placed at the right margin.` `
For example, the input
^EQ I (3.1a)

x = f(y/2) + y/2

^EN

produces the output

` ` ` ` ` `There is also a shorthand notation so
in-line expressions
like
can be entered without
.EQ
and
.EN.` `
We will talk about it in section 19.` `