The concurrent versions system (CVS) (www.cvshome.org) provides a convenient means by which programmers may obtain the latest (or any previous) version of the PSI3 source from the main repository, add new code to the source tree or modify existing PSI3 modules, and then make changes and additions available to other programmers by checking the modifications back into the main repository. CVS also provides a ``safety net'' in that any erroneous modifications to the code may be easily removed once they have been identified. This section describes how to use CVS to access and modify the PSI3 source code. (Note that compilation and installation instructions are given in a separate document.)