Application hooks

In app.cecil:

abstract object generic_app;

The generic_app object is a place to put all default behavior that can be customized by individual applications.

concrete object app isa generic_app;

The app object can be referred to in libraries to get access to application-specific behavior.

Library code should add default behavior to generic_app, but send messages to app to access it.

Individual applications can then add objects *in between* app and generic_app, using extend declarations to add inheritance links. E.g.

concrete object my_app isa generic_app;
extend app isa my_app;
... lots of application-specific methods and fields specialized on my_app,
    in some cases overriding the default implementations on generic_app;

Less nicely, they could add methods directly on app itself, if they know that no later app will want to reuse this code but override some of it.