Bring on the fnk

fnk_examples.clj

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

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

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

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

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