As mentioned above, SWI-Prolog contains two special modules. The 
first one is the module system. This module contains all 
built-in predicates. Module system has no import module. 
The second special module is the module user. This module 
forms the initial working space of the user. Initially it is empty. The 
import module of module user is system, making 
all built-in predicates available.
All other modules import from the module user. This 
implies they can use all predicates imported into user 
without explicitly importing them. If an application loads all modules 
from the
user module using use_module/1, 
one achieves a scoping system similar to the C-language, where every 
module can access all exported predicates without any special 
precautions.