(def a-var 1)

  < 1 >
  (alter-var-root
    #'a-var
    (fn [old]
      (Thread/sleep 10000)
      (inc old))))

  blocking call for 10 seconds
  < 2 >
2
(def a-var 1)

  < 1 >
  (alter-var-root
    #'a-var
    (fn [old]
      (Thread/sleep 10000)
      (inc old))))

  blocking call for 10 seconds
  < 2 >
2