split-with.clj

(defn my-split-with [pred coll]
  (let [p (promise)
        passes ((fn step [c]
                  (lazy-seq
                    (if-let [[x & xs] (seq c)]
                      (if (pred x)
                        (cons x (step xs))
                        (do (deliver p c) nil))
                      (do (deliver p nil) nil))))
                coll)]
    [passes
     (lazy-seq (do (dorun passes) @p))]))