a rule for Prolog predicate half_sister_of(X,Y)?
<p>The kb is </p>

<pre><code>parent_of(john,ann).
parent_of(mary,ann).
parent_of(mary,sylvia).
parent_of(brian,sylvia).

female(sylvia).
female(ann).


?-Half_sister_of(X,Y).
X=ann
X=sylvia ;

X=sylvia
X=ann 
</code></pre>