# System Macros/Fundamentals

### The ConTeXt version

\contextversion contains the ConTeXt version string. If you need to make sure you are running under ConTeXt, check for this macro. It is not defined in syst-gen.tex but in context.tex. This is because syst-gen.tex is sometimes loaded under Latex.

The expansion of the macro is something like this:

### Conditional execution of engine-specific code

There are a few macrosthat delimit code that is executed conditionally. \beginTEX and \beginETEX are mutually exclusive, depending on whether or not the format file was compiled under an e-TeX-enabled executable. A typical way of setting up your code to use e-TeX where available is like this:

\beginTEX
\def\ifundefined#1%
{\expandafter\ifx\csname#1\endcsname\relax}
\endTEX

\beginETEX \ifcsname
\def\ifundefined#1%
{\unless\ifcsname#1\endcsname}
\endETEX


Code delimited by \beginOMEGA ... \endOMEGA is only executed if ConTeXt runs under Omega, \beginXETEX ... \endXETEX only if XeTeX is used, ..., and ignored otherwise.

The optional argument after the \begin... can be used to give information to the viewer: the example above will print the following string to the terminal:

system (E-TEX) : [line 833] \ifcsname


Because modules can be used in various contexts, we want to be able to prevent macro files from being loaded more than once. This can be done using:

\abortinputifdefined\command


where \command is a command defined in the module to be loaded only once.

For example, syst-gen.tex implements \writestatus, and therefore it starts with:

\abortinputifdefined\writestatus


Actually you don't need this macro for modules, since \usemodule does it's own bookkeeping. It is intended for files that are loaded via the TeX primitive \input.

### Protecting internal macros

We can shield macros from users by using some special characters in their names. Some characters that TeX normally does not consider to be letters (and therefore used) are: @, ! and ?. Before and after the definition of protected macros, we have to change the <catcode> of these characters. This is done by \unprotect and \protect, for instance:

\unprotect
\def\!test{alfa}
\protect


The newly defined command \!test can of course only be called upon when we are in the \unprotect'ed state, otherwise TeX reads the command \!, followed by the word test and probably complains loudly about not being in math mode.

The protection/unprotection commands can be nested (unlike \makeatletter in LaTeX). This nesting is a convenience, since it allows one to use the protection pair regardless of whether protection is already turned on.

When the nesting becomes deeper than one level, the system reports the current protection level.

It is a good habit to always start your macro files with \unprotect and end them with \protect.