text: 4.12 Universal Restrictions 
  All of the restrictions we have created so far have been existential restrictions (defined using the some 
  DL keyword). Existential restrictions specify the existence of at least one relationship along a given 
  property to an individual that is a member of a specific class (specified by the filler). However, existential 
  restrictions do not mandate that the only relationships for the given property that can exist must be to 
  individuals that are members of the specified filler class. 
  
  For example, we could use an existential restriction hasTopping some MozzarellaTopping to 
  describe the individuals that have at least one relationship along the property hasTopping to an 
  individual that is a member of the class MozzarellaTopping. This restriction does not imply that all of 
  the hasTopping relationships must be to a member of the class MozzarellaTopping. To restrict the 
  relationships for a given property to individuals that are members of a specific class we must use a 
  universal restriction. Universal restrictions correspond to the symbol ∀ in First Order Logic. They 
  constrain the relationships along a given property to individuals that are members of a specific class. For 
  example, the universal restriction ∀ hasTopping VegetableTopping describes the individuals all of 
  whose hasTopping relationships are to members of the class VegetableTopping — the individuals do 
  not have a hasTopping relationship to individuals that aren’t members of the class 
  VegetableTopping. 