NavigableMap<String, String> map = ImmutableSortedMap.of("a", "4", "b", "9");
    EntryTransformer<String, String, String> concat =
        new EntryTransformer<String, String, String>() {
          @Override
          public String transformEntry(String key, String value) {
            return key + value;
          }
        };
    NavigableMap<String, String> transformed = transformEntries(map, concat);

    NavigableMap<String, String> map = ImmutableSortedMap.of("a", "4", "b", "9");
    EntryTransformer<String, String, String> concat =
        new EntryTransformer<String, String, String>() {
          @Override
          public String transformEntry(String key, String value) {
            return key + value;
          }
        };
    NavigableMap<String, String> transformed = transformEntries(map, concat);