 
 public protocol MigratorType {
     init(persistence: Persistence)
-    var persistence: Persistence { get }
+    var persistence: Persistence { get set }
     func isMigrationRequired() -> Bool
     func attemptMigration() throws
 }

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

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