solution for http://www.4clojure.com/problem/82#prob-title task

word_chain.clj

(defn one-or-no-letter-diff? [a b]
  (loop [xs (seq a)
         ys (seq b)]
    (cond
     (= xs ys) true
     (and (empty? xs) (= (count ys) 1)) true
     (and (empty? ys) (= (count xs) 1)) true
     (= (first xs) (first ys)) (recur (rest xs) (rest ys))
     :else (or (= (rest xs) ys) (= (rest ys) xs) (= (rest xs) (rest ys))))))

(defn path-exists? [coll word]
  (if (empty? coll) true
    (let [mutable-words (filter (partial one-or-no-letter-diff? word) coll)]
      (some #(path-exists? (disj coll %) %) mutable-words))))

(defn word-chain? [coll]
  (if (some #(path-exists? (disj coll %) %) coll) true false))