code: public static <E, C extends Counter<E>> Counter<E> L2NormalizeInPlace(Counter<E> c) {
      return multiplyInPlace(c, 1.0 / L2Norm(c));
    }
docs: L2 normalize a counter in place.
  
  @param c
  The {@link Counter} to be L2 normalized. This counter is modified
  @return the passed in counter l2-normalized