flex Expert System Toolkit - LPIS

2. Frames and Inheritance 15
So, whenever this rule is fired, the attribute methane_level of the frame
mine is given the current value slight.
Linking Frames
In the previous section we looked at the form and content of frames, which
are structures for representing objects. The frame system, however, provides
more than a data storage mechanism: by allowing objects to be linked to
each other, it enables a frame hierarchy to be established.
The links between frames determine the overall structure of the frame
hierarchy. Each link links a parent-frame to a child-frame. The child-frame
can be thought of as a specialisation of the parent-frame, or, the parentframe
as a generalisation of the child-frame.
A child-frame can inherit values (both default and current) from any of its
parent-frames, which in turn can inherit values from their parent-frames,
and so on. In this way, information filters down from the frames at the top
of the hierarchy to those leaf nodes at the bottom of the hierarchy.
This allows the distribution of information without duplication.
Let us consider a small section of the animal kingdom as below.
Animal
Mammal
Carnivore
Rodent
Feline
An arrow pointing from one frame to another indicates a parent-frame to
child-frame link in the hierarchy.
The KSL frame declarations for the above diagram are as follows:
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