code: private static void initMap() {
      // Go through all the enum constants and build a map of
      // LayoutVersion <-> EnumSet of all supported features in that LayoutVersion
      for (Feature f : Feature.values()) {
        EnumSet<Feature> ancestorSet = map.get(f.ancestorLV);
        if (ancestorSet == null) {
          ancestorSet = EnumSet.noneOf(Feature.class); // Empty enum set
          map.put(f.ancestorLV, ancestorSet);
        }
        EnumSet<Feature> featureSet = EnumSet.copyOf(ancestorSet);
        featureSet.add(f);
        map.put(f.lv, featureSet);
      }
      
      // Special initialization for 0.20.203 and 0.20.204
      // to add Feature#DELEGATION_TOKEN
      specialInit(Feature.RESERVED_REL20_203.lv, Feature.DELEGATION_TOKEN);
      specialInit(Feature.RESERVED_REL20_204.lv, Feature.DELEGATION_TOKEN);
    }
docs: Initialize the map of a layout version and EnumSet of {@link Feature}s
  supported.