code: @Override
      public void addSubPropertyOfAxiom(DataPropertyExpression dpe1, DataPropertyExpression dpe2) throws InconsistentOntologyException {
          checkSignature(dpe1);
          checkSignature(dpe2);
          dataPropertyAxioms.addInclusion(dpe1, dpe2);
      }
docs: Normalizes and adds a data subproperty axiom
  <p>
  SubDataPropertyOf := 'SubDataPropertyOf' '(' axiomAnnotations
  subDataPropertyExpression superDataPropertyExpression ')'<br>
  subDataPropertyExpression := DataPropertyExpression<br>
  superDataPropertyExpression := DataPropertyExpression
  <p>
  implements rule [D1]:<br>
  - ignore the axiom if the first argument is owl:bottomDataProperty
  or the second argument is owl:topDataProperty<br>
  - replace by a disjointness axiom if the second argument is owl:bottomDataProperty
  but the first one is not owl:topDataProperty<br>
  - inconsistency if the first is  owl:topDataProperty but the second is owl:bottomDataProperty
  
  @throws InconsistentOntologyException