Bonus: existential reasoning

Ontop includes a tree-witness query rewriting algorithm implementing existential reasoning.

To enable it, in Protégé go to Preferences -> Ontop reasoner tab and enable the option reasoning over anonymous individuals.

Then, restart the reasoner and try the following query that looks for people supervised by some Professor:

PREFIX : <http://example.org/voc#>
SELECT ?x {
   ?x :isSupervisedBy [ a :Professor ] .
}

Note that there is no mapping assertion for the property isSupervisedBy. However, the ontology states that for every GraduateStudent there must exist some Professor so that the student isSupervisedBy the professor. Ontop can thus infer that all GraduateStudent are answers to the query, which is then rewritten as the following equivalent query prior to evaluation:

PREFIX : <http://example.org/voc#>
SELECT ?x {
   ?x a :GraduateStudent .
}