That callback doesn't actually pass the fieldset down, because it hasn't been built yet. So:
```coffee
$ ->
  $(document).on 'has_many_add:before', '.has_many', (e)->
    if $(@).children('fieldset').length > 9
      alert "you reached the maximum number of items"
      return false
```
Unfortunately, that doesn't actually prevent the new fieldset from being added. So now the question I've been avoiding: how do we conditionally add the fieldset based on what the hook returns?

If there isn't a good way to do that, I think I should remove the before-add hook because having it implies functionality that we don't provide.