code: 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);
    }
docs: Return the l2 norm (Euclidean vector length) of a Counter.
  <i>Implementation note:</i> The method name favors legibility of the L over
  the convention of using lowercase names for methods.
  
  @param c
  The Counter
  @return Its length