The macro defclass is used to define a new named class. The
definition of a class includes the following:
The name of the new class. For newly defined classes this is a
proper name.
The list of the direct superclasses of the new class.
A set of slot specifiers. Each slot specifier includes the
name of the slot and zero or more slot options. A slot option
pertains only to a single slot. If a class definition contains two slot
specifiers with the same name, an error is signaled.
A set of class options. Each class option pertains to the
class as a whole.
The slot options and class options of the defclass form
provide mechanisms for the following:
Supplying a default initial value form for a given slot.
Requesting that methods for generic functions be automatically
generated for reading or writing slots.
Controlling whether a given slot is shared by instances of the class
or whether each instance of the class has its own slot.
Supplying a set of initialization arguments and initialization
argument defaults to be used in instance creation.
Indicating that the metaclass is to be other than the default.
Indicating the expected type for the value stored in the slot.
