Fibonacci sequence with offsets and weights

fibonacci.rkt

;; Fibonacci with offsets and weights
;; F(0) = A
;; F(1) = B
;; F(n) = xF(n-2) + yF(n-1)

(define (weighted-fibonacci n A B x y)
  (cond
    [(= n 0) A]
    [(= n 1) B]
    [else
     (local
       [(define (fib-adder a b counter)
       (cond [(= n counter) (+ (* x a) (* y b))]
             [else (fib-adder b (+ (* x a) (* y b)) (add1 counter))]))]
       (fib-adder A B 2))]))
       
; (build-list 20
;   (lambda (n) (weighted-fibonacci n 4 6 2 -1)))
; -> '(4 6 2 10 -6 26 -38 90 -166 346 -678 1370 -2726 5466 -10918 21850 -43686 87386 -174758 349530)