1.2.4 Cross reference among modules

Whenever a module uses the definition in other modules, it must explicitly import those modules. The keyword for importation is either ``includes'' or ``needs''. On the other hand, to enable another module to import itself and make part of the definition visible to it, the module must explicitly export with the keyword ``introduces''.

The scheme is similar to packages in Java, where a unit uses other units, and defines whatever is visible to the outside in its interface. This explicit declaration ensures modularity.