def fib: Stream[Long] = {
    def next(n1: Long, n2: Long): Stream[Long] = {
        n2 #:: next(n1+n2, n1)
    }
    next(1,1)
}

def toInf(n:Int):Stream[Int] = n #:: toInf(n+1)
def sieve(s: Stream[Int]): Stream[Int] =
    s.head #:: sieve(s.tail filter {case n => n % s.head != 0})
val primes = sieve(toInf(2))

def sqrtStream(x: Double): Stream[Double] = {
    def improve(guess:Double) = guess - (guess*guess - x)/(2*guess)
    def guesses: Stream[Double] = 1 #:: (guesses map improve)

    guesses
}