The first one is the module system . This module contains all built-in predicates described in this manual.
Module system has no default module assigned to it.
The second special module is the module user . This module forms the initial working space of the user. Initially it is empty. The default module of module user is system , making all built-in predicate definitions available as defaults. Built-in predicates thus can be overruled by defining them in module user before they are used.