// why does this not compile?
// Book implements BookRenderable
// so why does the compiler complain when Library.books is a [Book] and LibraryRenderable is looking for [BookRenderable] ??

protocol LibraryRenderable {
    var address : String? { get }
    var books : [BookRenderable]? { get }
}

protocol BookRenderable {
    var title : String? { get }
    var author : String? { get }
}

struct MockLibrary : LibraryRenderable {
  let address = "123 Main Street"
  let books = [MockBook(), MockBook()]
}

struct MockBook : BookRenderable {
  let title : String? = "Ender's Game"
  let author : String? = "Orson Scott Card"
}

class Library : LibraryRenderable {
  var address : String?
  var books : [Book]?  // <---- compiler complains "does not conform to LibraryRenderable"
}

class Book : BookRenderable {
  var title : String?
  var author : String?
}