When using multiple files how to save to specific one?

In the example you can open multiple files. When you run save(), how do I specify where it goes?

```
nconf.file('path/to/your/config.json');
// add multiple files, hierarchically. notice the unique key for each file
nconf.file('user', 'path/to/your/user.json');
nconf.file('global', 'path/to/your/global.json');

nconf.set('foo','bar');

nconf.save(); // Where is 'foo' saved?
```