public function clearToken($service)
    {
        // get previously saved tokens
        $tokens = Session::get($this->sessionVariableName);

        if (array_key_exists($service, $tokens)) {
            unset($tokens[$service]);
        }

        // Replace the stored tokens array
        Session::set($this->sessionVariableName, $tokens);

        // allow chaining
        return $this;
    }