flex Expert System Toolkit - LPIS

2. Frames and Inheritance 26
Universal Defaults
Flex supports the notion of universal default values. These are default
values which may be inherited by any frame in the hierarchy.
For example, in a system representing physical objects the notion of weight
is universal. It is the product of the object's volume and density.
Rather than have a weight slot in each frame of the hierarchy, it would be
more sensible to have a single global definition of weight which is universally
accessible.
This is accomplished by having a special frame called root, which is always
considered when inheriting values. The inheritance algorithm can be
directed to visit the root frame either before or after visiting any ancestor
frames.
Singular Versus Multiple Inheritance ?
Whenever a frame system allows for values to be inherited, there is the
possibility for alternative answers according to where the inheritance comes
from. This is in many ways similar to Prolog itself, which allows for
alternative answers to the same query.
The plurality of inheritance within flex can either be singular (the default)
or multiple. In both cases the search stops as soon as the first value is
found.
For singular inheritance there is a commitment to this first value, and no
others will ever be considered.
For multiple inheritance, however, alternative values can be obtained by
backtracking using the inheritance algorithm, which is a Prolog program.
In reality, the singular inheritance algorithm is the same as the multiple
inheritance algorithm except that it terminates with a cut (!) to avoid any
potential backtracking.
Frame Relationships
In its default setting, the only relationship between frames is the AKO (akind-of)
hierarchy which defines how values are to be inherited. In general,
though, it would be of great benefit to be able to define other relationships
between frames, such as all tigers can hunt humans.
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