Q:

Visual Metaphor for Inversion

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.

A:

If an icon doesn't jump to mind for you, then there probably isn't one that will have obvious meaning to your users either. You're better off using a text label for this. 
If it must be an icon, then it doesn't matter much what it is, since users won't be able to guess it any way, but at least try to make it visually distinct and memorable if not particularly intuitive. MS Access uses a funnel to represent "filtering." Maybe use an upside-down or white-on-black funnel for inversion? (An X'ed-out funnel means "don't filter").
Whatever. Like I said, it doesn't matter much.

