I was trying to avoid adding more nested arrays than necessary, but perhaps I was being too clever. 

Don't you think we should support simple array syntax like these?
```php
$rules = array(
    'required' => 'foo'
);
```
or even:
```php
$rules = array(
    'required' => array('foo', 'baz')
);
```
Your example is definitely easier to parse, but a bit more nested. Thoughts?