;;;; Instantiation of a skeleton expression 
;;;  with respect to a dictionary

(define (instantiate skeleton dictionary)
  (define (walk skel)
    (cond ((match-variable? skel)
	   (instantiate-variable skel
				 dictionary))
	  ((pair? skel)
	   (cons (walk (car skel))
		 (walk (cdr skel))))
	  (else skel)))
  (walk skeleton))

(define (instantiate-variable var dictionary)
  (let ((entry
	 (dictionary-lookup var dictionary)))
    (if entry
	(value-in-entry entry)
	(error "No value for " var))))


#|
;;; For example

(instantiate '(a (?x ?y) ?y)
	     '((?y c) (?x b)))
;Value: (a (b c) c)
|#