NSMutableSet vs Set

Should the reference to NSMutableSet at the top of pg 193 be a reference to a Swift Set?

This is one of those tricky edges between Cocoa/Objective-C and Swift. The return type of mutableSetValueForKey(_:slight_smile: is NSMutableSet, but if you were writing a Swift model object (like Teacher in figure 11.4) in practice you would probably use the Set type so that you could specify the type that it contains (Set).