I just arrived here to find the same thing.
Wanted to find all books with no comments
Although as you say Jackie, it's simple to type **:false - I really don't hang on to that kind of info in my brain
When presented with an empty field and need to try and remember what :,\.;/@ none alphanumeric characters I used 6months ago, well it just is not going to happen.
Like Samy, it would be nice to have the option in a simple dropdown, esp for none python users and ordinary people.