docs: public static void printSVMLightFormat(PrintWriter pw, ClassicCounter<Integer> c, int classNo) {
      Integer[] features = c.keySet().toArray(new Integer[c.keySet().size()]);
      Arrays.sort(features);
      StringBuilder sb = new StringBuilder();
      sb.append(classNo);
      sb.append(' ');
      for (int f: features) {
        sb.append(f + 1).append(':').append(c.getCount(f)).append(' ');
      }
      pw.println(sb.toString());
    }