I think you mentioned earlier that this function will throw an exception if the rule_id doesn't exist, right?

If that's the case, what I'd do is:

```
try
{
    $this->nestedset_rules->delete($rule_id);
}
catch (\specific\exception $e)
{
    throw new \phpbb\boardrules\exception\out_of_bounds(array('rule_id', 'INVALID_ITEM'));
}
```