How to do basic optimisation using loco

I am trying to do a basic example of optimisation using loco.

I have a vector of costs the index of which corresponds to the integer value of a number of slots and would like to minimize the sum of the costs for a distinct subset of the slots.

Please see my attempt below, which fails to work because there is no "link" between the picked slots and the costs.

(def costs [10 10 20 20 30 30 40 40 10 10])

(let [slot-vars (for [i (range 5)] ($in [:slot i] 1 10))
      cost-vars (for [i (range 10)] ($in [:cost i] 10 40))]
  (solution
   (concat
    slot-vars
    cost-vars
    [($distinct (for [i (range 5)] [:slot i]))]
    (for [i (range 5)]
      ($= [:cost i] (get costs i))))
   :minimize (apply $+ (for [i (range 5)] [:slot i]))))