struct URL {
    var scheme: String?
    var user: String?
    var password: String?
    var host: String?
    var port: Int?
    var path: String?
    var query: String?
    var fragment: String?

    var percentEncodedUser: String? {
        get { ... }
        set { ... }
    }

    // ...

    init(scheme: String? = nil, user: String? = nil, password: String? = nil, host: String? = nil, port: Int? = nil, path: String? = nil, query: String? = nil, fragment: String? = nil) {
        self.scheme = scheme
        self.user = user
        self.password = password
        self.host = host
        self.port = port
        self.path = path
        self.query = query
        self.fragment = fragment
    }

    init?(string: String) {
        // ...
    }
}