(define pop!
       (lambda (ra-stack)
	 (if (empty-ra-stack? ra-stack)
	     (error "POP!: attempted pop from an empty stack")
	     (begin
	       (node-set-element! ra-stack (- (height ra-stack) 1))
	       (node-set-rest! ra-stack (nodes-down 2 ra-stack))
	       ra-stack))))