blackbox.clj

(let [[a b c] (repeatedly gensym)
      e1 [a b]
      e2 [a c]]
  (fn [x]
    (let [y (conj x e1 e1 e2)
          diff (- (count y) (count x))]
      (condp = diff
        1 :map
        2 :set
        (if (= (first y) e2)
          :list
          :vector)))))

;;;My solution is similar but quite different too:

(defn tipe [s]
  (let [sym1  (gensym) sym2 (gensym) 
        sym3  (gensym) s12  (conj s {sym1 sym2}) 
        s1221 (conj (conj s {sym2 sym1}) s12)]
  (if (get s12 sym1) :map
      (if (get s12 {sym1 sym2}) :set      
          (if (= (first s1221) s12) :list :vector)))))