     private let kAPIBaseURLString = "https://api.foursquare.com/v2/"
 
     let session: NSURLSession
-    let accessToken: String
+    let accessToken: String?
+    let clientId: String?
+    let clientSecret: String?
     let version: String
 
     init(accessToken: String, version: String = "20150723") {

            delegate: nil,
            delegateQueue: NSOperationQueue.mainQueue())
        self.accessToken = accessToken
        self.version = version
        super.init()
    }


        // Add necessary parameters
        var parameter = parameter
        if self.accessToken != nil {
            parameter["oauth_token"] = self.accessToken
        }
        else if self.clientId != nil && self.clientSecret != nil {
            parameter["client_id"] = self.clientId
            parameter["client_secret"] = self.clientSecret
        }
        parameter["v"] = self.version

        let request: NSMutableURLRequest
