code: @Override
      public Node<OWLClass> getTopClassNode() {
          au.csiro.ontology.Node top = getTaxonomy().getTopNode();
          return nodeToOwlClassNode(top);
      }
docs: Gets the <code>Node</code> corresponding to the top node (containing
  <code>owl:Thing</code>) in the class hierarchy.
  
  @return A <code>Node</code> containing <code>owl:Thing</code> that is the
  top node in the class hierarchy. This <code>Node</code> is
  essentially equal to the <code>Node</code> returned by calling
  {@link #getEquivalentClasses(org.semanticweb.owlapi.model.OWLClassExpression)}
  with a parameter of <code>owl:Thing</code>.