maybe it was even better to call [canBeDisabled()](http://api.symfony.com/2.2/Symfony/Component/Config/Definition/Builder/ArrayNodeDefinition.html#method_canBeDisabled) on the ArrayNodeDefinition like this:

```php
->arrayNode('log')
    ->canBeDisabled()
    ->children()
        ->scalarNode('format')
            ->defaultValue('default')
        ->end()
    ->end()
->end()
```

This adds the possibility to use all the following formats:

```yaml
log: true

log: false

log: ~

log:
    enabled: ...
    format: ...
```