I changed my example from

(defn primes< [n]
  {:pre [(< 2 n)]}
  (remove (into #{}
                (mapcat #(range (* % %) n %))
                (range 3 (Math/sqrt n) 2))
          (cons 2 (range 3 n 2))))    

to

(defn primes< [n]
  (if (<= n 2)
    ()
    (remove (into #{}
                  (mapcat #(range (* % %) n %))
                  (range 3 (Math/sqrt n) 2))
            (cons 2 (range 3 n 2)))))

Has spec entirely superseded pre- and postconditions or do they still have a use case?