   ///
   /// Return the NSManagedObjectModel for this `ModelVersion`.
   public func managedObjectModel() -> NSManagedObjectModel {
-    let momURL = modelBundle.url(forResource: versionName, withExtension: "\(ModelVersionKey.mom)", subdirectory: momd)
+    let momURL = modelBundle.url(forResource: versionName, withExtension: "\(ModelVersionExtension.mom)", subdirectory: momd)
 
     /**
      As of iOS 11, Apple is advising that opening the .omo file for a managed object model is not supported, since the file format can change from release to release

  /// Protocol `ModelVersion`.
  ///
  /// Model file name.
  var momd: String { return "\(modelName).\(ModelVersionKey.momd)" }

}



import CoreData

private enum ModelVersionExtension {
  static let momd = "momd"
  static let omo  = "omo"
  static let mom  = "mom"
