This library module provides a Prolog unit-test framework, initially developed by Jan Wielemaker for SWI-Prolog. The code and documentation was subsequently adapted for SICStus Prolog by SICS. The module is a third-party product, and not part of SICStus Prolog proper.
Automatic testing of software during development is probably the most important Quality Assurance measure. Tests can validate the final system, which is nice for your users. However, most (Prolog) developers forget that it is not just a burden during development.