text: Our current description of CheesyPizza says that if something is a CheesyPizza it is*necessarily*a 
  Pizza and it is*necessary*for it to have at least one topping that is a kind of CheeseTopping. Now 
  consider some random individual. Suppose that we know that this individual is a member of the class 
  Pizza. We also know that this individual has at least one kind of CheeseTopping. However, given our 
  current description of CheesyPizza this knowledge is not sufficient to determine that the individual is a 
  member of the class CheesyPizza. To make this possible we need to change the conditions for 
  CheesyPizza from*necessary*conditions to*necessary AND sufficient*conditions. This means that not 
  only are the conditions*necessary*for membership of the class CheesyPizza, they are also*sufficient*to 
  determine that any random individual that satisfies them must be a member of the class CheesyPizza. 
  
  A class (such as all the classes we have defined so far) that only has necessary conditions is called a 
  *primitive class*. A class that has necessary and sufficient conditions is known as a*defined class*. In order 
  to convert necessary conditions to necessary and sufficient conditions, the conditions must be moved 
  from under the SubClass Of header in the class description view to be under the Equivalent To header. 
  This can be done with the menu option: Edit>Convert to defined class. 