/// Here's a definition of a `Countdown` sequence that serves as its own /// iterator. The `makeIterator()` method is provided as a default /// implementation. /// /// struct Countdown: Sequence, IteratorProtocol { /// var count: Int /// /// mutating func next() -> Int? { /// if count == 0 { /// return nil /// } else { /// defer { count -= 1 } /// return count /// } /// } /// } /// /// let threeToGo = Countdown(count: 3) /// for i in threeToGo { /// print(i) /// } /// // Prints "3" /// // Prints "2" /// // Prints "1"