Number padder in Lisp

number-pad.lisp

(defun pad(num)
  (concatenate 'string "0" num))

(defun should-pad(num)
  (= (length num) 1))

(defun apply-padding(num) 
    (cond
        ((should-pad num) (pad num))
        (t num))) 

(defun pad-list(nums) 
  (mapcar #'apply-padding nums) )

(print 
  (pad-list
    (list "1" "19" "2" "3" "55" "4" "12")))