A query is exactly the “opposite” of a fact—a Horn clause with no head:
mammal(horse) ←. or mammal(horse). ——a fact
← mammal(horse). ——a query or goal
A slightly more complicated example is the following.
Given the rules
legs( x, 2 ) ← mammal( x ), arms( x, 2 ).
legs( x, 4 ) ← mammal( x ), arms( x, 0 ).
mammal( horse ).
arms( horse, 0 ).
If we supply the query
← legs( horse, 4 ).
then applying resolution using the second rule, we get
legs(x,4) ← mammal(x), arms(x,0), legs(horse,4).
Now, to cancel the statements involving the predicate legs from each side, we have to match the variables x to horse, so we replace x by horse:
legs(horse,4)←mammal(horse),arms(horse,0),legs(horse,4).
and cancel to get the subgoals
← mammal( horse ), arms( horse, 0 ).
Now we apply resolution twice more using the facts mammal(horse) and arms(horse,0) and cancel:
mammal( horse ) ← mammal( horse ), arms( horse, 0 ).
← arms( horse, 0 ).
arms( horse, 0 ) ← arms( horse, 0 ).
←.
Since we have arrived at the empty statement, our original query is true.