In traditional Prolog systems the predicate space was flat. This approach is not very suitable for the development of large applications, certainly not if these applications are developed by more than one programmer. In many cases, the definition of a Prolog predicate requires sub-predicates that are intented only to complete the definition of the main predicate. With a flat and global predicate space these support predicates will be visible from the entire program.
For this reason, it is desirable that each source module has it's own predicate space. A module consists of a declaration for it's name, it's public predicates and the predicates themselves. This approach allow the programmer to use short (local) names for support predicates without worrying about name conflicts with the support predicates of other modules. The module declaration also makes explicit which predicates are meant for public usage and which for private purposes. Finally, using the module information, cross reference programs can indicate possible problems much better.