; Point class -- has 4 properties to represent a point in space coordinates  
;  and if it is currently occupied.  
  
( defclass point()  
  (  
   ( x :accessor point-x :initarg :x )  
   ( y :accessor point-y :initarg :y )  
   ( z :accessor point-z :initarg :z )  
   ( occupied :accessor point-occupied :initarg :occupied :initform 'nil )  
  )  
)