@jonathanong When we using rework, we can call `.functions` with multiple time, if functions not global, rework render the code

```javascript
css.use(rework.function({
  func-in-first-call: function(){},
  func2-in-first-call: function(){},
}).use(rework.function({
  func-in-second-call: function(){}
});
```

```css
func-in-first-call(func-in-second-call(func2-in-first-call(10px)))
```

will fail.

You can see the unit test:

https://github.com/bolasblack/rework/commit/5de6d8228bd3c78555332cf465267479dd298846#diff-fdacade56a5f218dd71703fab45cbd7aR221