docs: function addonsManager_categories(aSpec) {
      var spec = aSpec || { };
  
      var categories = this.getElements({
        type: "categories",
        subtype: spec.attribute,
        value: spec.value
      });
  
      if (categories.length == 0)
        throw new Error(arguments.callee.name + ": Categories could not be found.");
  
      return categories;
    }