unconj.clj

(defn unconj [coll]
        (when (seq coll)
          (let [x ((if (vector? coll) peek first) coll)]
            [(cond
               (list? coll) (rest coll) 
               (vector? coll) (subvec coll 0 (dec (count coll)))
               (set? coll) (disj coll x)
               (map? coll) (dissoc coll (x 0)))
             x])))

;; forall coll, (seq coll) => (= (apply conj (unconj coll)) coll)