shorthand syntax for checking if _id value is missing

In pipeline update specification it is common to check if a field doesn't exist and the way to tell if you're doing an upsert would be to check if _id doesn't exist. I'm proposing a shorthand syntax expression $onInsert or ifUpsert (exact work TBD) which would be short for:

 {$eq:[ {$type:"$_id"}, "missing" ] }