"Protocol Extension" in the Swift Programming: The Big Nerd Ranch Guide (2nd Ed.) e-book: This challenge is unique in that it does not have a specific problem or solution. Instead, it is an encouragement to spend some time reading interfaces written by the Swift team at Apple. Remember that you can Command-click on a type, function, method, or even operator to jump to a view in Xcode that shows you how the element is declared. You first encountered the map(_:) method in Chapter 13, where you called it on arrays. map(_:) is not just a method on arrays, though. map(_:) is defined in a protocol extension on all Sequences by the Swift standard library. The Swift standard library contains a large number of properties and methods provided by protocol extensions. Many of them also include where clauses that restrict their use based on various criteria. The Swift standard library uses many of the advanced features you have learned about. It may be difficult to read, especially at first, and especially if Swift is your first exposure to programming or generics. However, it is worth investing some time to look at how the library is organized. Try Command-clicking on Sequence in your playground and skimming through some of the extensions defined there. See if you can figure out what some of the where clauses mean. Do some experiments and explore!