Rick Block and I spent some time discussing the syntax for intersections (see [[Wikipedia:Category intersection]] or the shortcut [[WP:CI]]). Since there can be more than two categories involved, and you'd still want to pipe how it was displayed, we came up with: [[:Intersection:Actors::Americans::Living people|Living American actors]] I'm not certain that category unions are nearly as critical. Since all these "index" categories (Americans, Actors, Poets, Writers, etc...) would be fully populated, they would already contain the union of all their sub-categories. The only thing missing would be unions of only some of the subcategories, for instance, the union of Comedy films and Drama films, or Poets and Novelists. For most topics these do not seem very significant. There are several ways intersections could be implemented. We've sketched out 3 alternative approaches at [[WP:CI]]. -- Samuel Wantman