build-vocab.clj

(defn assoc-if-absent [m k f]
  (if (m k) m (assoc m k (f k))))

(defn tally-sent [vocab sent]
  (reduce
    (fn [res [before word after]]
      (-> res
          (assoc-if-absent word new-word-info)          
          (update-in [word] tally-usage before after)))
    vocab
    (partition 3 1 sent)))

(defn build-vocab [sents]
  (vals (reduce tally-sent {} sents)))