docs: public static <E, C extends Counter<E>> double L2Norm(C c) {
      double lenSq = 0.0;
      for (E key : c.keySet()) {
        double count = c.getCount(key);
        if (count != 0.0) {
          lenSq += (count * count);
        }
      }
      return Math.sqrt(lenSq);
    }