Short, concise description of the idea
Allow people to set custom security to "people from Group A, but not if they're also on Group B".
Full description of the idea
Say you want to make a post and show it to all your friends except a few in particular -- maybe your parents are on your friendslist or something, and you'd rather not expose them to details of your personal life. Currently, in order to do that, you need to create a separate friends group, something like "not-parents". Even if you already have a "parents" filter just for them. It seems like it would not require much effort to make the custom filtering interface be fully boolean, with friends groups being applied using "OR" and "AND NOT".
An ordered list of benefits
An ordered list of problems/issues involved
An organized list, or a few short paragraphs detailing suggestions for implementation