module b {
       yang-version 1.1;
       namespace "urn:example:b";
       prefix "b";

       revision 2015-04-04;
       revision 2015-01-01;

       typedef myenum {
         type enumeration {
           enum zero; // added in 2015-01-01
           enum one;  // added in 2015-04-04
         }
       }

       container x {  // added in 2015-01-01
         container y; // added in 2015-04-04
       }
     }