A handy macro for writing the bodies of compare methods for user classes.
Returns the result of comparing the two objects by comparing the results of
calling each of accessors, in order, on the objects. Despite the name, an
accessor can actually be any function on the class in question; it can also
be a symbol, which will be used to access the slot via slot-value. For
example, if class frob has accessor frob-foo and slot bar:

(defmethod compare ((f1 frob) (f2 frob))
(compare-slots f1 f2 #'frob-foo 'bar))