foo.clj

(def base-car #{:four-wheels})

(defmulti what-is-this identity)
(defmethod what-is-this #{:four-wheels} [_] (println "this is a base car"))
(defmethod what-is-this #{:four-wheels :mine} [_] (println "this is a base car of mine"))
(defmethod what-is-this #{:four-wheels :v8} [_] (println "this is a big car"))
(defmethod what-is-this #{:four-wheels :v8 :mine} [_] (println "this is a big car of mine"))
(defmethod what-is-this :default [_] (println "this is not a car"))


user=> (what-is-this base-car)
this is a base car
nil
user=> (conj base-car :mine)
#{:four-wheels :mine}
user=> (what-is-this *1)
this is a base car of mine
nil
user=> (conj base-car :suprise-tractor-hitch)
#{:four-wheels :suprise-tractor-hitch}
user=> (what-is-this *1)
this is not a car
nil
user=>