static String getSubsetPrefix(final PdfDictionary dic) {
        if (dic == null)
            return null;
        String s = getFontName(dic);
        if (s == null)
            return null;
        if (s.length() < 8 || s.charAt(6) != '+')
            return null;
        for (int k = 0; k < 6; ++k) {
            char c = s.charAt(k);
            if (c < 'A' || c > 'Z')
                return null;
        }
        return s;
    }