Before you throw me out into the cold with your bold assertions that this is not programming related, please hear me out.
I'm looking for a visual metaphor (Icon) to suggest the idea of inversion of a filter. So if a user has a filter which reduces a list of 10 items to 4, I want a button that will allow the user to invert the filter to display only the other 6.
Another wrinkle is that the UI will also have a button for removing the filter nearby.
While this certainly isn't a nuts-and-bolts programming question, I think it's relevant to the process of software creation. As a developer it's relatively easy to construct the mechanisms to perform complex filters, but it's all for naught if normal users find the presentation confusing.