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#lang racket
(define/contract (prefixes-div-by5 A)
(-> (listof exact-integer?) (listof boolean?))
(define ns (make-vector (length A) #false))
(for/fold ([acc 0])
([b (in-list A)]
[i (in-naturals)])
(let ([test-val (remainder (+ (* 2. acc) b) 5)])
(when (= 0 test-val) (vector-set! ns i #true))
test-val))
(vector->list ns))
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