protocol Encodable {
    var encoded: Decodable? { get }
}

protocol Decodable {
    var decoded: Encodable? { get }
}


extension Sequence where Iterator.Element: Encodable {
    var encoded: [Decodable] {
        return self.filter({ $0.encoded != nil }).map({ $0.encoded! })
    }
}


extension Sequence where Iterator.Element: Decodable {
    var decoded: [Encodable] {
        return self.filter({ $0.decoded != nil }).map({ $0.decoded! })
    }
}