Trigger/hook system for classes
                   ===============================

Problem:
========

A class B might want to be informed about changes of objects of class
A.  As class A might have been designed before class B, we cannot
simply use trigger methods of class A to implement the behavior
desired by class B, as class A would not know what class B wants to
be done.

Furthermore, it would be bad design to implement such methods as
methods of class A, as they concern class B.

Solution:
=========

Class A defines hooks for certain changes on its data. In its trigger
methods, it causes execution of the methods registered with said
hooks (by calling a GEAS function with the hook name).

Example of usage:
=================

[NOTE: The classes used in this example might not exist or might be
implemented differently. This just an example!]

In the employee class, there is a field that stores the employee's
income. The employee class defines a hook that is to be called when
the income field is changed.

In a trigger method (onChange), this hook is called with the employee
ID and the old and new income as parameters.

The department class defines a field that stores the total income of
all employees in that department. It defines a function that looks up
the employee ID, checks the department and updates the total income
field of the employee's department accordingly, and registers this
function with the hook.

Maintanability issues:
======================
[TODO: rewrite/rename this section]

Allowing registering functions with hooks at run-time would raise
several issues:
 - persistence:
   such registration might have to be stored between sessions.
 - deterministic behavior:
   it would be difficult to ensure deterministic behavior, as run-time
   registration might be unpredictable.

Therefore, hooks and functions connected with them should be defined
in the class definition store (GCD files, XML, database, etc.).

Implementation:
===============

[TODO: The syntax for describing hooks has yet to be defined.]

GEOR issues:
============

GEOR has to track which hooks a class provides and which functions are
connected with a hook.

It has to maintain the latter whenever packages are added, updated, or
removed. This might also happen at run-time.

The above-mentioned will be requested by GEAS when executing hooks.

GEAS issues:
============

GEAS has to cause execution of the registered functions whenever a
hook is triggered.