Slight documentation bug on SWI Prolog website:
On the page
https://eu.swi-prolog.org/pldoc/doc/_SWI_/library/plunit.pl
The top says:
Unit testing environment for SWI-Prolog and SICStus Prolog.
For usage, please visit section('packages/plunit.html').
That URL should either point to
https://www.swi-prolog.org/pldoc/doc/_SWI_/library/plunit.pl
itself or maybe
https://www.swi-prolog.org/pldoc/doc_for?object=section(%27packages/plunit.html%27)