code: private au.csiro.ontology.Node getNode(OWLClass oc) {
          final Object id = getId(oc);
  
          final au.csiro.ontology.Node n;
  
          if(id instanceof String) {
              n = getTaxonomy().getNode((String)id);
          } else if(NamedConcept.TOP.equals(id)) {
              n = getTaxonomy().getTopNode();
          } else if(NamedConcept.BOTTOM.equals(id)) {
              n = getTaxonomy().getBottomNode();
          } else {
              throw new RuntimeException("Unexpected id "+id);
          }
  
          return n;
      }
docs: Returns the {@link au.csiro.ontology.Node} for an {@link OWLClass}.
  If the owl class is anonymous then it throws a {@link RuntimeException}.
  
  @param oc
  @return