overview of finiteness proof Gerog comment "not helpful", adding more intuitions of "closed forms"
package Main
object Twice {
def apply(x: Int): Int = x * 2
def unapply(z: Int): Option[Int] = if (z % 2 == 0) Some(z / 2) else None
}
object TwiceTest extends App {
val x = Twice(10)
x match { case Twice(n) => Console.println(n) } //prints 21
}