Request: functionality to report WHICH expected variables are missing

in SEND13 rule, we check whether all the "expected" variables are in the dataset.
The rule works well, but there is no way yet (as far as I know) to report which expected variable is missing (or a list of them).
All we can do for now, is in "Output Variables" set "$expected_vars".
It would be nice if we could do something like "$expected_vars notin $vars"

The same functionality of course would also apply to "required variables" or even to "permissible variables".