gistfile1.clj

(defprotocol Fly
  (fly [this] "Method to fly"))

(defn bird-fly [this]
  (str (:name this) " flies..."))

(with-precondition! #'bird-fly
  :not-crow
  (fn [this] (not= (:name this) "Crow")))

(defrecord Bird [name species]
  Fly
  (fly [this] (bird-fly this)))

(def crow (Bird. "Crow" "Corvus corax"))
(fly crow) ; => Raises an exception due to precondition violation.