Guide to LaTeX (4th Edition) (Tools and Techniques

8.5. Some comments on user-defined structures 203
Local definitions are really not to be recommended. It is far better to
place all user definitions at the beginning, in the preamble, where they
are most easily visible. A local redefinition of a standard command can
lead to puzzling effects that are hard for others, or even for the author,
to trace down. Of course, some special effects might just be wanted.
8.5.5 Order of definitions
User-defined structures may be nested within one another. If one user
definition contains another user-defined structure, it is often the case that
the inner one has already been defined. However, this is not necessary.
User definitions may contain other user structures that are defined afterwards.
What is important is that the other structure is defined before the
first command is invoked.
For example, a normal sequence of definitions would be
\newcommand{\A}{defa}
\newcommand{\B}{defb}
\newcommand{\C}{\A \B}
where \C is defined in terms of \A and \B. However, it is also permissible
to write
\newcommand{\C}{\A \B}
normal text, but without calling \C
\newcommand{\B}{defb}
\newcommand{\A}{defa}
further text with any number of calls to \A, \B and \C
8.5.6 Nested definitions
User definitions may be nested inside one another. A structure such as
! \newcommand{\outer}{{\newcommand{\inner}...}}
is permissible. The command that is defined with the name {\inner} is valid
and known only within the command {\outer}, according to the remarks on the
scope of definitions on the facing page. Although the T E X macros make copious
use of nested command definitions to limit the lifetime of temporary commands,
it is not recommended to nest L AT E X definitions excessively since it is too easy
to lose track of the bracketing. A forgotten bracket pair will produce an error
message on the second call to the outer command, since the inner definition still
exists as a leftover from the first call. Nevertheless, here is an example:
\newcommand{\twentylove}
{{\newcommand{\fivelove}
{{{\newcommand{\onelove}
{I love \LaTeX!}%
\onelove\ \onelove\ \onelove\ \onelove\ \onelove}}}
\fivelove\\ \fivelove\\ \fivelove\\ \fivelove}}