code: public static String qualify (String bundle, String key)
      {
          // sanity check
          if (bundle.indexOf(QUAL_PREFIX) != -1 ||
              bundle.indexOf(QUAL_SEP) != -1) {
              String errmsg = "Message bundle may not contain '" + QUAL_PREFIX +
                  "' or '" + QUAL_SEP + "' [bundle=" + bundle +
                  ", key=" + key + "]";
              throw new IllegalArgumentException(errmsg);
          }
          return QUAL_PREFIX + bundle + QUAL_SEP + key;
      }
docs: Returns a fully qualified message key which, when translated by some other bundle, will know
  to resolve and utilize the supplied bundle to translate this particular key.