Merge sort implementation in lisp

mergesort.lisp

(defun merge-sort (lst)
  (let ((size (length lst)))
    (if (= size 1)
        lst
        (progn
          (let ((seq1 (merge-sort (subseq lst 0 (floor (/ size 2)))))
                (seq2 (merge-sort (subseq lst (floor (/ size 2)) size))))
            (merge-it seq1 seq2))))))


(defun merge-it (l1 l2)
  (let ((new-arr (make-array (+ (length l1)
                                (length l2))
                             :fill-pointer 0)))
    (loop for idx from 0 to (+ (length l1) (length l2)) do
         (let ((x (car l1))
               (y (car l2)))
           (when (and (not (null x)) (not (null y)))
             (if (<= x y)
                 (progn
                   (setf l1 (cdr l1))
                   (vector-push x new-arr))
                 (progn
                   (setf l2 (cdr l2))
                   (vector-push y new-arr))))))
    (mapcar #'(lambda (e) (vector-push e new-arr)) (append l1 l2))
    (coerce new-arr 'list)))