code: private Variants parseEras(JsonObject calendar) {
      JsonObject node = resolve(calendar, "eras");
  
      JsonObject abbr = resolve(node, "eraAbbr");
      JsonObject narrow = resolve(node, "eraNarrow");
      JsonObject wide = resolve(node, "eraNames");
  
      return new Variants(
          new String[] { string(abbr, "0"), string(abbr, "1") },
          new String[] { string(narrow, "0"), string(narrow, "1") },
          new String[] { },
          new String[] { string(wide, "0"), string(wide, "1") });
    }
docs: Parse era information (BC, AD).