So define the following node mutators:
(define node-set-element!
(lambda (node new-element)
(vector-set! node 0 new-element)))
(define node-set-rest!
(lambda (node new-rest)
(vector-set! node 1 new-rest)))
To decrement a stack's height:
(node-set-element! ra-stack (- (height ra-stack) 1))
To "jump around" a stack's top node:
(node-set-rest! ra-stack (nodes-down 2 ra-stack))