//: Playground - noun: a place where people can play import UIKit protocol SelfRewritable { var id: Int { get set } init(id: Int) mutating func increaseID() } extension SelfRewritable { mutating func increaseID() { self = Self(id: self.id + 1) } } class User: SelfRewritable { var id: Int required init(id: Int) { self.id = id } } var user = User(id: 0) user.increaseID() user.id