Transpose a matrix in Clojure

transpose.clj

(defn square-matrix?
  [mat]
  (apply =
         (count mat)
         (for [r mat] (count r))))

(defn transpose
  [mat]
  {:pre [(square-matrix? mat)]}
  (loop [res [],
         mat mat]
    (if (empty? (first mat))
      res
      (recur (conj res
                   (vec (for [r mat] (first r))))
             (for [r mat] (rest r))))))

Or even this:

(defn square-matrix?
  [mat]
  (apply =
         (count mat)
         (for [r mat] (count r))))

(defn transpose
  [mat]
  {:pre [(square-matrix? mat)]}
  (apply map vector mat))