there is what I mean:

this is idiomatic Lisp:

  (define (keep pred l)
     (cond ((null? l) '())
        ((pred (car l))
         (cons (car l) (keep pred (cdr l))))
        (else (keep pred (cdr l)))))

this is nonsense:

  (defn keep
    "Returns a lazy sequence of the non-nil results of (f item). Note, this means false return values will be included.  f must be free of side-effects."
  {:added "1.2"
   :static true}
  ([f coll]
   (lazy-seq
    (when-let [s (seq coll)]
      (if (chunked-seq? s)
        (let [c (chunk-first s)
              size (count c)
              b (chunk-buffer size)]
          (dotimes [i size]
            (let [x (f (.nth c i))]
              (when-not (nil? x)
                (chunk-append b x))))
          (chunk-cons (chunk b) (keep f (chunk-rest s))))
        (let [x (f (first s))]
          (if (nil? x)
            (keep f (rest s))
            (cons x (keep f (rest s))))))))))

When one breaks the underlying ideas he ruins the spell..