You seem to like the `__` vars :p 

I would call this `is_missing_data` or something a little more descriptive than `__chk`. The same goes for `__val` I think `missing_data_val` would be better.

remember these variables are only defined in the scope of this function so they do not form part of the API and therefore do not need to be 'hidden' from the user by using `_` or `__`.