(let [total (fn total [items discount]
             (reduce :+ 0.0 (map
                              (fn [{quantity quantity unitPrice unitPrice}]
                                (* (float (* .quantity .unitPrice)) (- 1.0 .discount)))
                              .items)))]
  [(total [{quantity 3 unitPrice 20} {quantity 2 unitPrice 15}] 0.1)
   (total [{quantity 2 unitPrice 15}] 0.0)
   (total [] 0.5)])