code: public Set<Pair<Unifier, SemanticDifference>> getMultiUnifierWithSemanticDiff(ReasonerAtomicQuery parent){
          return this.getMultiUnifier(parent, UnifierType.SUBSUMPTIVE).stream()
                  .map(u -> new Pair<>(u, this.getAtom().semanticDifference(parent.getAtom(), u)))
                  .collect(Collectors.toSet());
      }
docs: Calculates:
  - subsumptive unifier between this (child) and parent query
  - semantic difference between this (child and parent query
  @param parent parent query
  @return pair of subsumptive unifier and semantic difference between this and parent query