Artificial Intelligence and Soft Computing: Behavioral ... - Arteimi.info

6.4 Illustrating Backtracking by
Flow of Satisfaction Diagrams
To explain how the system answers these queries, we have to learn a very
useful phenomenon, called backtracking.
Let us now concentrate on the query: Teacher (X)?
Since
Teacher (X) ←
Person (X),
Writes-on (X, board, classhour). (10)
to satisfy the Goal: Teacher (X), one has to satisfy the sub-goals: Person (X),
Writes-on (X, board, classhour). Now, PROLOG searches a sub-goal Person( )
for the predicate Person (X). At clause 5, it finds a match and X is instantiated
to john (fig. 6.2 (a)). PROLOG puts a marker at clause 5. Now, it continues
searching Writes-on (john, board, classhour) in the remaining clauses. But it
fails to find so, since Writes-on (john, board, classhour) is at 2 nd position in
the list of clauses (fig. 6.2 (b)). So, it has to trace back above the marker place
(fig. 6.2(c)) and then ultimately it finds Writes-on (john, board, classhour)
(fig. 6.2(d)). Since the sub-goals are succeeded, the goal also succeeds,
yielding a solution: Teacher (john). The concept of backtracking is illustrated
below with the help of flow of satisfaction diagrams [2] fig. 6.2(a) to (d)).
Teacher (X)
Person (X)
Writes-on (X, board, classhour)
Fig. 6.2 (a): Unification of the first sub-goal.
X = john, by (5)