code: public static <K, E> void addToList(Map<K, List<E>> map, K k, E e)
      {
          getList(map, k).add(e);
      }
docs: Adds the given element to the list that is stored under the
  given key. If the list does not yet exist, it is created and
  inserted into the map.
  
  @param <K> The key type
  @param <E> The element type
  @param map The map
  @param k The key
  @param e The element