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A class describes a collection of objects
that share behavior. The objects described by a class
are called the instances of the class. The class
specifies the names of the slots that the
instance has, although it is up to the instance to
populate these slots with particular values.
The class also specifies the methods that can be
applied to its instances. Slot values can be anything,
but method values must be procedures.
Classes are hierarchical. Thus, a class can be a subclass of another class, which is called its superclass. A subclass not only has its own direct slots and methods, but also inherits all the
slots and methods of its superclass. If a class has a
slot or method that has the same name as its
superclass's, then the subclass's slot or method is the
one that is retained.
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