Optimization in KDE Base
QueryEngine: Avoid using rdfs:label in comparison/literal term
Virtuoso does not seem to be able to infer subPropertyOf relations. This
renders the LiteralTerm completely useless.
Instead we just use a generic variable for now. It might slightly
slower, but it is better than it not working. Everthing hinges on
Virtuoso bug - https://sourceforge.net/tracker/?func=detail&aid=3591024&group_id=161622&atid=820574