Example

Let family.pl consist of the KB

% male(P) is true when P is male
male(james1).
male(charles1).
male(charles2).
male(james2).
male(george1).

% female(P) is true when P is female
female(catherine).
female(elizabeth).
female(sophia).

% parent(C,P) is true when C has a parent called P
parent(charles1, james1).
parent(elizabeth, james1).
parent(charles2, charles1).
parent(catherine, charles1).
parent(james2, charles1).
parent(sophia, elizabeth).
parent(george1, sophia).