The example of writing an implementation of COALESCE made sense before the latest refactor. Now, COALESCE is implemented by:

```
class Coalesce(Func):
    function = 'COALESCE'
```
But that's not a very good example of the API. Is there a particular example that anyone can think of that would demonstrate the API appropriately? What is a SQL construct that you'd like to see implemented that hasn't previously been possible?