(define-struct-clause :food-box (food-type :form form)
  (unless (subtypep food-type 'edible)
    (compile-error form "~s requires edible type, not ~s" :food-box food-type))
  ^(food
    (:method set-food (me food)
      (unless (typep food ',food-type)
        (error "~s: requires ~s object, not ~s" %fun% ',food-type food))
      (set me.food food))))