 */
 class Migrator_v0_v1: MigratorType {
     
-    internal let persistence: Persistence
+    internal var persistence: Persistence
     required init(persistence: Persistence) {
         self.persistence = persistence
     }

public class CompositeMigrator: MigratorType {
    
    public var persistence: Persistence {
        preconditionFailure("No persistence here")
    }
    
    private let childMigrators: [MigratorType]
    public required init(persistence: Persistence) {
        self.childMigrators = [
            Migrator_v0_v1(persistence: persistence),

*/
class Migrator_v1_v2: MigratorType {
    
    internal var persistence: Persistence
    required init(persistence: Persistence) {
        self.persistence = persistence
    }
