private function _minify (& $css, $path) {
		$result = '';
		if (!is_callable(static::$MinifyCallable)) {
			$this->exception(
				"Configured callable object for CSS minification doesn't exist. "
				.'Use: https://github.com/mrclay/minify -> /min/lib/Minify/CSS.php'
			);
		}
		try {
			$result = call_user_func(static::$MinifyCallable, $css);
		} catch (\Exception $e) {
			$this->exception("Unable to minify style sheet ('$path').");
		}
		return $result;
	}