private static function set( $instance = false ) {
		// Make sure the plugin hasn't already been instantiated before.
		global $wp_plugins;
		if ( ! isset( $wp_plugins ) ) {
			$wp_plugins = new \stdClass();
		}
		// Get the fully qualified parent class name and instantiate an instance of it.
		$called_class = get_called_class();
		$plugin_name  = strtolower( $called_class );
		if ( empty( $instance ) || ! is_a( $instance, $called_class ) ) {
			$wp_plugins->$plugin_name = new $called_class();
		} else {
			$wp_plugins->$plugin_name = $instance;
		}
	}