protected function loadPlugin($plugin)
    {
        $pluginName = get_class($plugin);

        $apiVersion = self::API_VERSION_0_9;
        if ($plugin instanceof PicoPluginInterface) {
            if (defined($pluginName . '::API_VERSION')) {
                $apiVersion = $pluginName::API_VERSION;
            } else {
                $apiVersion = self::API_VERSION_1_0;
            }
        }

        if (!isset($this->plugins[$apiVersion][$pluginName])) {
            // PicoDeprecated currently supports all previous API versions
            $this->plugins[$apiVersion][$pluginName] = $plugin;
        }
    }