(: calc-tax-bracketed-rate (-> Nonnegative-Real BracketSpec Nonnegative-Real))
(define (calc-tax-bracketed-rate income brackets)
  (let calc-impl ([income income]
                  [brackets brackets]
                  [tax 0.0])
    (match-let ([(cons lo rate) (car  brackets)]
                [(cons hi    _) (cadr brackets)])
      (if (or (< income lo) (null? brackets))
          ;; stopping conditions:
          ;;  - income < lo   =>  we're one bracket too high
          ;;  - null? bracket =>  we ran out of brackets
          tax
          (calc-impl income
                     (cdr brackets)
                     (+ tax (* rate (- (min (+ hi 1) income) lo))))))))