#| BEGIN (Write your solution here) |#
(define (sign-change-detector x y)
  (if (< y 0)
      (if (< x 0)
          0
          -1)
      (if (< x 0)
          1
          0)))
(define zero-crossings
  (stream-map sign-change-detector sense-data (cons-stream (stream-car sense-data) sense-data)))
#| END |#