mock

struct Point3D {
    var x: Int
    var y: Int
    var z: Int
}

protocol Point3DGenerator {
    mutating func next() -> Point3D
}

struct Point3DGeneratorImpl {
    var current: Point3D
    init(_ cur: Point3D) {
        current = cur
    }
    mutating func next() -> Point3D {
        current.x += 2
        current.y -= 2
        current.z *= 2
        return current
    }
}

func generateThree(generator: Point3DGenerator) -> (Point3D, Point3D, Point3D) {
    return (generator.next(), generator.next(), generator.next())
}