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 ...