;; list-names-in-order:  BST -> ListOfString
;; consumes a binary search tree and produces a list of the names of the taxpayers,
;; sorted in order by ascending taxpayer id.
(define (list-names-in-order a-bst)
  (cond [(boolean? a-bst) empty]
        [(taxpayer? a-bst) (append 
                            (list-names-in-order (taxpayer-left a-bst))
                            (list (taxpayer-name a-bst))
                            (list-names-in-order (taxpayer-right a-bst)))]))