+/** This class simplifies interaction with a group of radio buttons:
+ * one, and only one, can be selected.
+ * The idea is that you register a bunch of radio buttons with
+ * an accompanying value. Then you can get or set the active button with a
+ * single function call.
+ * It is necessary to also group a family of radio buttons in the
+ * corresponding .fd file in order to unset the previously chosen button
+ * when a new one is selected.
+ */
+class RadioButtonGroup {