Brent's Feeds and Folders problem. See: http://roopc.net/posts/2015/brents-feeds-and-folders/

FeedsAndFolders.swift

protocol Feed: Equatable {
    var url: String { get }
}

func ==<T: Feed>(lhs: T, rhs: T) -> Bool {
    return (lhs.url == rhs.url)
}

protocol Folder {
    typealias FeedType: Feed
    var feeds: [FeedType] { get }
    func addFeeds(feedsToAdd: [FeedType])
}

struct LocalFeed: Feed {
    var url: String
    init(url: String) {
        self.url = url
    }
}

class LocalFolder: Folder {
    var feeds: [LocalFeed] = []
    func addFeeds(feedsToAdd: [LocalFeed]) {
        for oneFeed in feedsToAdd {
            if !feeds.contains(oneFeed) {
                feeds += [oneFeed]
            }
        }
    }
}

let folder = LocalFolder()
folder.addFeeds([ LocalFeed(url: "1"), LocalFeed(url: "2") ])
folder.addFeeds([ LocalFeed(url: "2"), LocalFeed(url: "3") ])
print(folder.feeds.count) // 3