He's using the "copy on write" feature of the file system. So it should leave A_1 untouched, creating a new copy for A_0's modifications. More info: https://dev

He's using the "copy on write" feature of the file system. So it should leave A_1 untouched, creating a new copy for A_0's modifications. More info: https://developer.apple.com/documentation/foundation/file_system/about_apple_file_system https://developer.apple.com/documentation/foundation/file_sy...