throws Throwable
  public static Value str_rot13(String string)    throws Throwable  {    StringBuilder sb = new StringBuilder();    int len = string.length();    for (int i = 0; i < len; i++) {      char ch = string.charAt(i);      if ('a' <= ch && ch <= 'z') {        int off = ch - 'a';        sb.append((char) ('a' + (off + 13) % 26));      }      else if ('A' <= ch && ch <= 'Z') {        int off = ch - 'A';        sb.append((char) ('A' + (off + 13) % 26));      }      else {        sb.append(ch);      }    }    return new StringValueImpl(sb.toString());  }