This module defines complex types for sets of things. These sets are elements called "set", with a required "id" attribute, an optional "label" attribute, and one ore more attributes named after the elements that are members of the set. The value of these latter attributes are ID references. Set elements can be annotated (i.e. enclose "meta" elements), in which case they need to have an "about" attribute, whose value matches that of the "id" attribute prefixed with a "#" symbol. Set elements are placed at the bottom of the container element that collects candidate members of the set. Hence, concrete instances of the set elements are defined in the modules that define the respective container elements.