как перезаписать (defun eval (expr)) функцию в LISP

Я новичок в программировании на LISP, и это конец семестра, и наш учитель попросил нас сделать этот проект, и я пытался его сделать, но я застрял, поэтому любая помощь будет оценена по достоинству. Проект состоит в том, чтобы написать eval (expr) функцию на Лиспе, чтобы перезаписать уже существующую функцию. вот подробности:

Описание проекта: элементы в арифметическом выражении, разделенные пробелами;

; Input:
;    1. The form of arithmetic expression given in prefix notation like LISP 
; Assumptions:
;    1. binary operations for +, -, *, and / 
;    2. integer division, no reals
;    3. an arithmetic expression occupies only one line
;    4. nested arithmetic expressions permitted
;    5. all given inputs are syntax correct
;    6. no need for error handling  

Я написал код, который может вычислять простые арифметические выражения, и он работает!! но я НЕ МОГУ заставить его работать с вложенными арифметическими операциями. Я думаю, что у меня проблема с частью рекурсии, я делаю что-то не так, но что именно idk :(

вот мой код:

; Assign a character string to a global variable input-prompt
; treat input-prompt as a constant global variable
(setf input-prompt "Please input an arithmetic expression: ")

(setf output-prompt "The value is: ")

(defun prompt-for-input (msg)
  (format t msg)
  (format t "~%"))   ; ~% new line

(defun prompt-for-output (msg)
  (format t msg))

(defun output-msg (result)
  (format t "~S" result) ; ~S takes the result into the print message
  (format t "~%"))

(defun eval (expr)
  (print "My EVAL Function is Working *_*")
  (if (numberp expr) expr)     
  (cond
    ((eq (car expr) '+)
      (if (and (numberp (cadr  expr)) 
               (numberp (caddr expr))) ;to check if we have simple expression
        (+ (cadr expr) (caddr expr)) ;in case both are numbers we add normally
        (if (not (numberp (cadr expr))) 
           ;in case the second argument is not number 
           ;we need to call eval again to check the expression 
          ((eval (cadr exp)))
          (if (not (numberp (caddr expr))) 
             ;in case the third argument is not a number 
             ;we need to call eval again to check the expression
            ((eval (caddr exp)))
            (0)))))
    ((eq (car expr) '-)
      (if (and (numberp (cadr  expr)) 
               (numberp (caddr expr))) ;to check if we have simple expression
        (- (cadr expr) (caddr expr)) ;in case both are numbers we add normally
        (if (not (numberp (cadr expr))) 
           ;in case the second argument is not number 
           ;we need to call eval again to check the expression 
          ((eval (cadr exp)))
          (if (not (numberp (caddr expr))) 
             ;in case the third argument is not a number 
             ;we need to call eval again to check the expression
            ((eval (caddr exp)))
            (0)))))
    ((eq (car expr) '*)
      (if (and (numberp (cadr  expr)) 
               (numberp (caddr expr))) ;to check if we have simple expression
        (* (cadr expr) (caddr expr)) ;in case both are numbers we add normally
        (if (not (numberp (cadr expr))) 
           ;in case the second argument is not number 
           ;we need to call eval again to check the expression 
          ((eval (cadr exp)))
          (if (not (numberp (caddr expr))) 
             ;in case the third argument is not a number 
             ;we need to call eval again to check the expression
            ((eval (caddr exp)))
            (0)))))
    ((eq (car expr) '/)
      (if (and (numberp (cadr  expr)) 
               (numberp (caddr expr))) ;to check if we have simple expression
        (/ (cadr expr) (caddr expr)) ;in case both are numbers we add normally
        (if (not (numberp (cadr expr))) 
           ;in case the second argument is not number 
           ;we need to call eval again to check the expression 
          ((eval (cadr exp)))
          (if (not (numberp (caddr expr))) 
             ;in case the third argument is not a number 
             ;we need to call eval again to check the expression
            ((eval (caddr exp)))
            (0)))))))

    ; it should have eval(expr) function which returns the value of the
    ; arithmetic expression
    ; for instance, 
    ; (+ 2 3) outputs 5
    ; (+ (* 3 2) (/ 4 2))) outputs 8
    ; (* (- 2 3) 5) outputs -5 

    ; driver accepts the input arithmetic expression
    ; evaluate the arithmetic expression
    ; output the reulst of the evaluation of the arithmetic expression
    ; execution is in the loop; to exit the loop; type cntrl-c

(defun driver ()
  (prompt-for-input input-prompt) 
     ;to print "Please input an arithmetic expression
  (let ((expression (read)))      
    (output-msg expression)
    (let ((result (eval expression)))
      (prompt-for-output output-prompt)  
      (output-msg result)))
  (driver))