The link on this page in the section titled ** 1.1 Pengine references** to sandbox.pl:
http://www.swi-prolog.org/pldoc/doc_for?object=section('packages/pengines.html')
The link generates a 404 error:
http://www.swi-prolog.org/pldoc/doc/home/vnc/prolog/lib/swipl/library/sandbox.pl