(defn- proper? [matrix]
  "Returns true if the matrix has the same number of elements for each row."
    (apply = (map #(count %) matrix)))

(defn get-row [matrix row]
  "Returns the nth row"
  {:pre [(proper? matrix) (< row (count matrix))]}
    (nth matrix row))

; ClojureMatrix.test.matrix

(def sample-matrix [[1 2 3] [4 5 6] [7 8 9]])
(deftest get-row-test
    (is (= [1 2 3] (get-row sample-matrix 0)))
    (is (= [7 8 9] (get-row sample-matrix 2)))
    (is (thrown? AssertionError (get-row sample-matrix 5))))