(defn zero-if-nil [m k]
  (let [x (m k)]
    (if (nil? x)
      0
      x)))

(defn train [model features]
  (loop [my-model model
         my-features features]
    (if (seq my-features)
      (recur (assoc my-model (first my-features) (+ 1 (zero-if-nil my-model (first my-features)))) (rest my-features))
      my-model)))