6.5 Module Compilation
One source file can contain several modules and one module
may spread over several files3.
The module structure is controlled by the module/1
directive which tells the compiler that all subsequent input
up to the end of file or another module directive will
be part of the given module.
When it encounters the module/1 directive,
the compiler first erases previous contents of this module,
if there was any, before starting to compile predicates
This means that if the contents of a module has to be
generated incrementally, the module directive cannot be used
because the previous contents of the module would be destroyed.
In this case the predicate compile(File, Module)
should be used.