/**
* Inverts the specified map: the values of the map become keys in the resulting map,
* and the keys become values.
*
* Behavior of duplicate values (key overlap) in the new map is undefined: it depends
* on the {@link Map.Entry} iterator of the supplied map.
*
* @param the map to be inverted. can be null
* @return a new map, never null, but possibly empty (and immutable).
*/
static <K,V> Map<V, K> invertMap(Map<K,V> map) {
   if (map == null) {
       return Collections.emptyMap();
   }
   return map.entrySet().stream().collect(Collectors.toMap(Map.Entry::getValue, Map.Entry::getKey));
}