text: 1. Select the class CheeseTopping from the class hierarchy. 
  
  2. Create a subclass of CheeseTopping called ProbeInconsistentTopping. 
  
  3. Click on the Add icon (+) next to the SubClass Of field in the Description view for 
  ProbeInconsistentTopping. 
  
  4. Select the Class hierarchy tab from the dialogue that pops up. This will bring up a small view that 
  looks like the class hierarchy tab you have been using to add new classes. Use this to navigate to and 
  select the class VegetableTopping. Click on OK. 
  
  5. Make sure to save your current ontology file. Now run the reasoner. You should see that 
  ProbeInconsistentTopping is now highlighted in red indicating it is inconsistent. 
  
  6. Click on ProbeInconsistentTopping to see why it is highlighted in red. Notice that at the top of the 
  Description view you should now see owl:Nothing under the Equivalent To field. This means that the 
  probe class is equivalent to owl:Nothing. The owl:Nothing class is the opposite of owl:Thing. 
  Whereas all individuals are instances of owl:Thing, no individual can ever be an instance of 
  owl:Nothing. The owl:Nothing class is equivalent to the empty set in set theory. 
  
  7. There should be a ? icon just to the right of owl:Nothing. As with any inference of the reasoner it is 
  possible to click on the new information and generate an explanation for it. Do that now, click on the ? 
  icon. This should generate a new window that looks like figure 4.20. The explanation is that 
  ProbeInconsistentTopping is a subclass of CheeseTopping and VegetableTopping but those 
  two classes are disjoint. 
  
  8. Click OK to dismiss the window. Delete the class ProbeInconsistentTopping by selecting it and then 
  clicking on the delete class icon at the top of the classes view (see figure 4.4). 
  
  9. Synchronize the reasoner. 