public static <E> MapPBag<E> empty(final PMap<E,Integer> map) {
		return new MapPBag<E>(map.minusAll(map.keySet()), 0); }
public static < E > MapPBag < E > empty ( final PMap < E , Integer > map ) { return new MapPBag < E > ( map . minusAll ( map . keySet ( ) ) , 0 ) ; }
@param <E>
@param map
@return a PBag backed by an empty version of map, i.e. by map.minusAll(map.keySet())