spec.cljc

(require '[clojure.spec :as s])

(derive ::x ::x-or-y)
(derive ::y ::x-or-y)

(defmulti multifn first)

(defmethod multifn ::x-or-y [_]
  (s/tuple keyword? number?))

(defmethod multifn ::z [_]
  (s/tuple keyword? string?))

(s/def ::my-spec
  (s/and (s/multi-spec multifn (fn [value tag]
                                 (assoc value 0 tag)))
         #(empty? (descendants (first %)))))

(s/exercise ::my-spec) ;; Only generates ::z :(