It would be very useful if you could filter not only by "IS" but also by "IS NOT". For example, I can filter in the inbox to only show certain labels. It would be helpful if I could filter to show what doesn't have that label. I imagine it working like this: If I select the filter once, a checkmark appears, and if I select it again, a red X appears, showing everything that doesn't have that label, the checkbox is empty again after the third click.