(define-datatype target target?
  (direct-target
    (expval expval?))
  (indirect-target
    (ref ref-to-direct-target?))
  )

(define expval?
  (lambda (val)
    (or (number? val) (procval? val))
    )
  )

(define ref-to-direct-target?
  (lambda (val)
    (and (reference? val) 
         (cases reference val
	    (a-ref (pos vec)
		(cases target (vector-ref vec pos)
		   (direct-target (v) #t)
		   (indirect-target (v) #f)
                  ))
           ))
    )
  )

(define primitive-deref deref)

(define deref 
  (lambda (ref)
     (cases target (primitive-deref ref)
        (direct-target (expval) expval)
        (indirect-target (ref-ref)
           (cases target (primitive-deref ref-ref)
              (direct-target (expval) expval)
              (indirect-target (p)
                (eopl:error 'deref "Illegal reference: ~s" ref-ref))
             ))
       )
    )
  )

(define primitive-setref! setref!)

(define setref!
  (lambda (ref expval)
     (let ((target-ref (cases target (primitive-deref ref)
                          (direct-target (aval) ref)
                          (indirect-target (aref) aref))))
        (primitive-setref! target-ref (direct-target expval))
       )
    )
  )