code: public static <K, V> void putAllIntoListMap(K key, Collection<? extends V> values, Map<? super K, List<V>> map) {
  		List<V> list = map.get(key);
  		if (list == null) {
  			list = Lists.newArrayList(values);
  			map.put(key, list);
  		} else {
  			list.addAll(values);
  		}
  	}
docs: Puts a value into a map that supports lists as values.
  The list is created on-demand.