Database and Expert System Technology - Information Systems Reengineering, Integration and Normalization - page 74

3. Frame-Based Systems
The main idea of a frame is to collect all information related to one concept in one
place. It attempts to reason about classes of objects by using “prototypical” repre-
sentations of knowledge that hold for the majority of cases. The intuition behind
the theory was that conceptual encoding in the human brain is less concerned with
defining strictly and exhaustively the properties that entities must possess in order
to be considered exemplars of some category, and more concerned with the salient
properties associated with objects that are somehow typical of their class (Jackson
1990 ). Figure 2.17 shows an example of a frame-based system based on the KAPPA
system (IntelliCorp 1994 ).
A frame is a complex structure that can store and represent knowledge by using
the “Slot and Filler” formalisms, as termed in (Forst 1987 ). A slot is an attribute that
describes a frame. A frame usually consists of a number of slots. A filler describes
values of a slot. A slot has only one filler. There are two types of frames in most
frame systems, a class and an instance frame. A class frame is a description of a
class of entities in the world. An instance frame is an intention description of an
individual entity in the world. For example, “Jack is a Human.” In this knowledge,
“Human” is generic knowledge and can be a class frame. “Jack” is an individual
object and can be an instance frame.
Frames are always linked into taxonomies by using two types of links: subclass
links and member links. A subclass link represents the generalization relationship
between class frames. Class frames can have subclass links to one or more other
class frames. For example (see Fig. 2.17 ), Sedans is a subclass of Autos. The mem-
ber link represents the class membership between instance frame and class frame.
Any instance frame can have a member link to one or more class frames. For ex-
ample, Tomscar is a member of Sedans. These links provide two standard interpre-
tations of the meaning of “is_a” links, such as “Jack is a Human” and “Human is
a Mammal” (see Fig. 2.15 . The “is_a” link supports inheritance for frame-based
systems. Most current frame-based systems support multiple inheritance. There are
two main problems that must be solved here. First, there is a need for the system to
distinguish between its own slots and those it has inherited, and to decide the prior-
ity of the two types of slot. The systems own slots will usually get higher priority
than inherited slots. For example (see Fig. 2.15 ), a human has two arms, but one-
arm-human is a human who only has one arm. The own slot property will overwrite
the inheritable property. Another example is “Bird is an animal.” “The locomotion
mode of animal is walking” but “Bird can not only walk but also fly.” The own slot
property and inheritable property must exist together. Second is a need to solve any
conflict problems between inherited slots. If inherited slots from different frames
have the same slot name there is a conflict. The general solution for this problem is
to keep only one slot from the highest priority inheritance frame, or to keep these
slots at different levels. The situation is similar to the first problem.
An important source of the expressive power of frame-based languages are the fa-
cilities that they provide for describing object attributes, called slots (Minsky 1975 ).
These facilities allow frames to include partial descriptions of attribute values, and
help preserve the semantic integrity of a system's knowledge base by constraining