248: Highlights

Class hierarchies reflect subclass–superclass relations among classes.
An instance can inherit instance variables and methods not only from the class to which it belongs, but also from all that class's superclasses.
When a subclass–superclass chain contains multiple methods with the same name, the one closest to the receiver in the subclass–superclass chain is the one that is employed. All others are overridden.
If you want to create a class hierarchy, then draw a diagram that reflects natural categories, and use the diagram to help ensure the following:
- There is no unnecessary duplication of instance variables or methods.
- Each instance variable and each method defined in a class is useful in all that class's subclasses.
If you want to define a predicate that tests instances for class membership, then instantiate the following method pattern for the lowest class in the class hierarchy that is a superclass of all classes whose instances are possible receivers of the predicate:
```
predicate name 
  ^ false 
```
and then instantiate the following method pattern for the class itself:
```
predicate name 
  ^ true 
```