Bring on the fnk

fnk_examples.clj

(defnk foo [x y [s 1]]
  (+ x (* y s)))

(foo {:x 2 :y 3 :s 2})
; ==> 8

;; 's' defaults to 1 
(foo {:x 2 :y 3})
; ==> 5

(foo {:x 2})
; ==> (Exception. "Missing argument :y ...")

(meta foo)
; ==> {:req-ks #{:x :y}}
;      :opt-ks #{:s}