docs: public static String readTag(Reader r) throws IOException {
      if ( ! r.ready()) {
        return null;
      }
      StringBuilder b = new StringBuilder("<");
      int c = r.read();
      while (c >= 0) {
        b.append((char) c);
        if (c == '>') {
          break;
        }
        c = r.read();
      }
      if (b.length() == 1) {
        return null;
      }
      return b.toString();
    }