View difference between Paste ID: tmJQ0p8j and rZDEEcDp

 (defun flatten (x &optional y)
   (declare (optimize (speed 3) (safety 0)))
   (cond
     ((null x)
      (cond
        ((null y) nil)
        ((listp (car y))
         (flatten (car y) (cdr y)))
        (t (cons (car y) (flatten (cdr y))))))
     ((listp (car x))
      (flatten (car x) (if y (list (cdr x) y) (cdr x))))
     (t (cons (car x) (flatten (cdr x) y)))))
 
 (defun flatten-1 (l)
   (declare (optimize (speed 3) (safety 0)))
   (cond
     ((null l) nil)
     ((atom l) (list l))
     (t (loop for a in l appending (flatten-1 a)))))
 
 (defun flatten-2 (l)
   (declare (optimize (speed 3) (safety 0)))
   (when l
     (if (atom l)
 	(list l)
 	(mapcan #'flatten-2 l))))
 
 (defun flatten-3 (x)
   (declare (optimize (speed 3) (safety 0)))
   (do ((a x))
       ((null a))
     (if (listp (car a))
 	(do ((b (car a) (cdr b))
 	     (c a)
 	     (d (cdr a)))
-  (print (- (get-internal-real-time) test)))
+	    ((null b)
-;; results
+	     (and (setf (cdr c) d) nil))
-;; 886
+	  (setf (car c) (car b)
-;; 2868
+		(cdr c) (cdr b)
-;; 1828
+		c (or (cdr c) c)))
 	(setf a (cdr a)))) x)
 
 ;; 353
 ;; 2056
 ;; 1230
 ;; 63
 
 (let ((test (get-internal-real-time)))
   (dotimes (i 1e6)
     (flatten '(1 2 (3 4) (5 (6 (7 8) 9 a (10))) b (10.5 (1.6) (1.7)) 11)))
   (print (- (get-internal-real-time) test))
   (setf test (get-internal-real-time))
   (dotimes (i 1e6)
     (flatten-2 '(1 2 (3 4) (5 (6 (7 8) 9 a (10))) b (10.5 (1.6) (1.7)) 11)))
   (print (- (get-internal-real-time) test))
   (setf test (get-internal-real-time))
   (dotimes (i 1e6)
     (flatten-1 '(1 2 (3 4) (5 (6 (7 8) 9 a (10))) b (10.5 (1.6) (1.7)) 11)))
   (print (- (get-internal-real-time) test))
   (setf test (get-internal-real-time))
   (dotimes (i 1e6)
     (flatten-3 '(1 2 (3 4) (5 (6 (7 8) 9 a (10))) b (10.5 (1.6) (1.7)) 11)))
   (print (- (get-internal-real-time) test)))