Page 145 - Intelligent Communication Systems

I 24 INTELLIGENT COMMUNICATION SYSTEMS

This means that if (P lt P 2,---, P n) is true, then Q is true. Equation (11) is called a
Horn clause. The following expressions are defined based on a Horn clause:

Equation (12) means that Q is true if (P lt P 2,.,., P n) is true. Equation (13) means that
P 2,...,
Q is unconditionally true. Equation (14) means it is a question whether (P lt
P n) is true or not. In other words, it is a goal clause. Equation (15) is a null clause.
A set composed of Eqs. (12), (13), (14), and (15) is called a Horn set. Using
a Horn set, a resolution is performed as follows: Make a goal clause one of the
parent clauses. Select a Horn clause expressed in Eq. (12) or Eq. (13) whose left
literal is matched to any of the literals in a goal clause. The literal of the goal clause
is replaced by the right part of the Horn clause (12) or (13). The operation contin-
ues until the result becomes a null clause. Here is an example of a resolution:

Example: The following Horn clauses are given.
Like(;t, y) 4- Man(jt), Beer(y)
Like(jt, y) <— Woman(;c), Apple(y)
Man(Ted)
Beer(Asahi) <—
Woman(Hannah) <—
Apple(Katie) <—
When a goal clause <— Like(Ted, Asahi) is given, The resolution in Figure 10.14
is conducted.
10.4.8 Application to Telecommunication Service

For example, consider directory service. The following Horn sets are defined to
solve the goal clause:
S e(S 0, 5) <- S(S Q, S,), V(S lt 5 2), 0(S 2, S)

S([x\S], 5) <-
V(S 0,S)*-AV(S 0,S l),V(S l,S)