protected function isOutputExpired() {
		if ($this->inputFiles->isEmpty()) {
			throw new OperationError('Nothing to assemble, you should add at least one item');
		}
		
		if (! is_file($this->outputFile)) {
			return true;
		}
		
		$outputMtime = $this->getFileMTime($this->outputFile);
		
		foreach ($this->inputFiles as $path) {
			if ($outputMtime < $this->getFileMTime($path)) {
				return true;
			}
		}
		
		return false;
	}