 Solution

The algorithm is $\boxed{\text{incorrect: it can return true for a non-entailed ground atom}}$. Its recursive calls return only Booleans. They do not return and propagate the first-order substitutions obtained while satisfying earlier rule premises.

Take distinct constants $a,b,c$, factsand the single Datalog ruleAsk for $H(c)$. The head matches the goal without binding $x$. In line 15, the recursive call for $P(x)$ succeeds by first-order unification with $P(a)$, but returns only true. The next recursive call still receives $Q(x)$ with $x$ unbound, and succeeds by first-order unification with $Q(b)$. Neither subgoal is the visited head $H(c)$, so the visited test does not stop these calls. Both recursive results are true, and the algorithm returns true for $H(c)$.

However, the least Herbrand model is just $\{P(a),Q(b)\}$. No single constant makes both $P(x)$ and $Q(x)$ true, so the rule cannot derive $H(c)$. The two individually successful premises need one consistent first-order substitution. A correct backward chaining procedure must thread answer first-order substitutions through the remaining premises and backtrack when they cannot be extended; forward chaining on ground Datalog instances is another way to avoid this error.