I screwed up here by prematurely merging in the previous change to add support for `@index`; we don't want to handle `@key` and `@index` as individual special case keywords; rather, we want to support _any_ data keyword that's prefixed with an `@`. This way, the helpers have full control over what data vars to add. 

So this rule needs to be changed to handle `@` followed by any valid identifier, not just `@key` and `@index` and whatever else might come in the future. Would you mind making this change?