Copying and Substitution Functions [These are system functions in Common Lisp. The system functions subst and sublis copy only as much structure as necessary.]

(defun copy-tree (z)
  (if (consp z)
      (cons (copy-tree (first z))
            (copy-tree (rest z)))
      z) )

; substitute x for y in z
(defun subst (x y z)
  (if (consp z)
      (cons (subst x y (first z))
            (subst x y (rest z)))
      (if (eql z y) x z)) )

; substitute in z with bindings in alist
(defun sublis (alist z)
  (let (pair)
    (if (consp z)
        (cons (sublis alist (first z))
              (sublis alist (rest z)))
        (if (setq pair (assoc z alist))
            (cdr pair)
            z)) ))