neighbors.clj

; Joy of Clojure pp85-86
(defn neighbors
          ([size yx] (neighbors [[-1 0] [1 0] [0 -1] [0 1]] size yx))
          ([deltas size yx]
             (filter (fn [new-yx]
                       (every? #(< -1 % size) new-yx))
                     (map #(map + yx %) deltas))))

; usage
(def matrix
             [[1 2 3]
              [4 5 6]
              [7 8 9]])

(map #(get-in matrix %) (neighbors 3 [0 0]))
;=> (4 2)

The key to understanding this is that it is method overloading by the arity of the arguments.

SO answer explaining arity overloading
Dave Newton: so you're either passing in a size and yx, or the delta and the same two things