Implement the concept of "questions" one can ask of an arbitrary entity (ad-hoc components)

Not entirely convinced that this design is the way to go, but am currently without better ideas.
Perhaps each "key" (answerer) could/should provide a dispatch function?

src/hottub/question.clj

@@ -0,0 +1,33 @@
+(ns hottub.question
+  (:require [clojure.set :as set]))
+
+(def question-registry (atom {}))
+
+(defn set-questions-for-key [key & questions]
+  (swap! question-registry 
+         #(reduce (fn [registry question] 
+                    (assoc registry question
+                           (if-let [keys (get registry question)]
+                             (conj keys key)
+                             #{key})))
+                  %1 questions)))
+
+(defn keys-for-question 
+  ([question] 
+     (or (get @question-registry question) #{}))
+  ([question entity]
+     (set/intersection (keys-for-question question) (set (keys entity)))))
+
+(defn questions [] (keys @question-registry))
+
+(defmacro defquestion [name [entity & rest-args] & impl]
+  `(do
+     (defmulti ~name [entity# ~@rest-args] (first (keys-for-question ~name entity#)))
+     (defmethod ~name :default [~entity ~@rest-args] ~@impl)))
+
+(set-questions-for-key :foo :bar :x)
+(set-questions-for-key :baz :bar :y)
+(keys-for-question :bar)
+(keys-for-question :bar {})
+(keys-for-question :bar {:foo 3})
+(questions)

src/hottub/util.clj

@@ -8,3 +8,12 @@
 
 (defn timestamp []
   (/ (System/nanoTime) 1000000))
+
+(defn ?assoc
+  "Same as assoc, b ut skip the assoc if v is nil"
+  [m & kvs]
+  (into m
+        (for [[k v :as e] (partition 2 kvs) :when (not (nil? v))]
+          (vec e))))
+
+