"That was like a crazy trust exercise." -Anna

@frozen is an attribute to structure and enum declaration to restrict the kind of changes you can make. This attriute is allowed only when compiling in library evolution mode.

@frozen struct Animal { ... }

To enable library evolution mode on the command line, pass the -enable-library-evolution option to the swift compiler. To enable it in Xcode, set the “Build Libraries for Distribution” build setting (BUILD_LIBRARY_FOR_DISTRIBUTION) to Yes, as described in Xcode Help.