Make facets optionally include OR exclude by the user

Problem/Motivation

A given facet must currently be either Include or Exclude, meaning that someone searching a site can only perform one of those functions. For example, on a digital repository site with a Genre facet that includes Photographs, Newspapers, and Theses, an end-user performing a search might want to search for both Photographs and Theses but not Newspapers (exclude Newspapers). Another end user might want to search for only Newspapers (include Newspapers). The current methods for configuring facets only offer one option -- either include or exclude.

Proposed resolution

Another facet configuration option (in addition to checkbox, dropdown, etc.) that exposes the choice of Include OR Exclude to the user, for example, in the form of +/- links.

Example of a Drupal 7 facet implementation: Drupal 7 search screenshot