gistfile1.clj

(defn pop-lax
   "Pop the vector, but given an empty vector, simply return it."
   [v]
   (if (seq v) (pop v) []))

(def ^{:doc "Add a vertex to the last curve."}
   append-vertex!
   ^{:actname "add vertex"}
   (fn [^Point2D wp]
      (dosync
         (let [old-curves (.curves @udata)
               front (pop-lax old-curves)
               end (or (peek) [])]
            (assoc-in-ref! udata [:curves]
               (conj (conj end wp) front))))))