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\Section{Macros and Definitions}
We have already shown one instance in which a \TeX\ control
sequence was defined or modified by the user.  This was
the use of |\font| to define a font-switching control
sequence.
However, one can also define a new \TeX\ control sequence
by using the primitive |\def|.  Such new control sequences
are called {\sl macros\/} as they are, in effect, short
abbreviations for longer streams of commands.
It is, of course, possible to define whole
families of macros similar in function to the macro packages
such as {\bf --me} or {\bf --mm} in \Troff/, or the `document
formats' such as {\bf @Make(Report)} of |Scribe|.  Knuth's
{\sl\TeX book\/} was produced using such a package\note
{This package is described in Appendix~E of that book.
Some of Knuth's macros were stolen for
this document.}, and the
numbered
section headings and footnotes for this document were produced
by the author's own macros such as the |\Section| macro seen in
the example at the beginning of the document.

However, 
even when using a very complete  macro package, one sometimes
wishes to define often-used sequences for a particular paper.
For example, this paper
uses a macro, |\Unix|, which is similar to |\TeX| but  produces
`\Unix' in 10-point small-caps font.  It was defined as follows:
\begintt
\font\csc=cmcsc10 % Small-caps font
\def\Unix{{\csc Unix}} % Extra grouping for font-change
\endtt
As a result, whenever |\Unix| is seen in the input
for this paper, \TeX\
substitutes the
sequence |{\csc Unix}|. 

As another example, one might wish to refer to the backslash
character many times in typewriter-font examples (for instance,
if one were writing a document about \TeX).  In this case, it
might be useful to define a control sequence such as |\\| as
follows:
\begintt
\def\\{\char`\\}
\endtt
After which an example like |\this\example\here| could be
typed as
\begintt
{\tt \\this\\example\\here}
\endtt

If you have a file containing a group of useful macros and definitions,
or even just a useful section of text that you wish to
repeat, you can use the \TeX\ primitive |\input|, which operates
much like the |.so| primitive of \Troff/.  For example, the
definitions of |\Section| and so on for this paper are
contained in a file called |paperhead.tex|; the first line
of this document is |\input paperhead|.
If the file to be input isn't in the current working directory,
a ``macro library'' directory is searched.  This allows commonly-used
macro packages, such as the American Mathematical Society's
{$\cal A$\kern-.1667em\lower.5ex\hbox{$\cal M$}\kern-.125em $\cal S$-\TeX}
package, to be publicly available with a minimum of fuss.

It is also possible to generate a separate runnable version of
\TeX\ that has a built-in set of macros other than those used
by plain \TeX.  One such system is the excellent 
L\kern-.2em\raise.3ex\hbox{\csc a}\kern-.09em\TeX\
document preparation system created by Leslie Lamport of
SRI International, which presents a far more complete system
for producing complex text documents than plain \TeX.

More complex macros with parameters, etc., are possible, but
are `big-league' material outside the scope of this paper.
The grouping features of \TeX, combined with a powerful
set of conditionals,
give \TeX\ a structured-language-like
appearance, compared to that of \Troff/.
The Truly Ambitious should read ``{\sl The \TeX book\/}'' by Donald
Knuth, which is the ultimate font of \TeX nical knowledge.
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