Clearly, if the last resolution step matched ¬ Grandparent(x, Harmonia) with a positive clause, that positive clause would carry a binding for x that would provide an answer to our question.
The technique of answer extraction augments the negated conclusion
with an Answer predicate to capture the desired binding. Thus, the
negated conclusion becomes:
¬ Grandparent(x, Harmonia) &or Answer(x)
The Answer predicate is ``invisible'' when counting the number of literals, so that a clause consisting only of the Answer predicate is recognized as □.
More generally, the conclusion can be augmented by an ``invisible''
predicate that is the positive form of the negated question predicate:
¬ Grandparent(x, Harmonia)
&or Grandparent(x, Harmonia)
This form can capture multiple variable bindings.