find-cycle

(defn find-cycle
  "finds a single cycle in the parents
   starting at the seed index"
  [a b seed]
  (loop [coll (mapv vector a b)
         cyl  #{}
         cur  (get-in coll [seed 0])]
    (let [idx (first (keep-indexed #(if (= (first %2) cur) %1) coll))]
      (if (nil? idx)
        (vec cyl)
        (recur (vec-remove coll idx)
               (into cyl (nth coll idx))
               (last (nth coll idx)))))))