CL-USER> (define-condition foo ()
               ((%bar :initarg :bar :allocation :class))
               (:default-initargs :bar 42))
FOO
CL-USER> (describe (make-instance 'foo))
#<FOO {1008BFF4B3}>
  [condition]

Slots with :CLASS allocation:
  %BAR                           = #<unbound> ;; why is it unbound?