text: **Exercise 13: Add a restriction to Pizza that specifies a Pizza must have a PizzaBase**
  
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  1. Select Pizza from the class hierarchy on the Classes tab. 
  
  2. Click on the Add icon (+) next to the SubClass Of field in the Description view for Pizza. 
  
  3. This will bring up a new window with several tab options to define a new restriction. Select the Object 
  restriction creator. This tab has the Restricted property on the left and the Restriction filler on the right. 
  
  4. Expand the property hierarchy on the left and select hasBase as the property to restrict. Then in the 
  Restriction filler on the right select the class PizzaBase. Finally, the Restriction type at the bottom should 
  be set to Some (existential). This should be the default so you shouldn’t have to change anything but 
  double check that this is the case. Your window should look like figure 4.16 now. 
  
  5. When your UI looks like figure 4.16 click on the OK button. That should close the window. Run the 
  reasoner to make sure things are consistent. Your main window should now look like figure 4.17. 