Structure "woodboard":

(defstruct woodboard
    (size 'twoXfour)
    (length 8)
    (strength 'medium)
)

Structure "balsa" inherits from "woodboard":

(defstruct (balsa (:include woodboard (strength 'light))))


Class "board":
(defclass board ()
    ((size :accessor board-size :initarg :size :initform 'twoXfour)
     (length :accessor board-length :initarg :length :initform 8)
     (strength :accessor board-strength :initarg :strength :initform 'medium)
    )
)

Class "balsaboard" has "board" as a superclass:

(defclass balsaboard (board)
    ((strength :initform 'light)))


Create an instance of woodboard:
> (setf b1 (make-woodboard))

Access the strength of b1:
> (woodboard-strength b1) --> MEDIUM

Create an instance of balsa:
> (setf b2 (make-balsa))

Values of fields for b2:
b2: size = twoXfour, length = 8, strength = light

Create an instance of class "balsaboard":
> (setf b3 (make-instance 'balsaboard))

Get the value of b3's strength:
> (board-strength b3) --> LIGHT