default Filterable<T> retainAll(final Stream<? extends T> stream){
  final Set<T> set=stream.collect(Collectors.toSet());
  return filter(null);
}

default Filterable < T > retainAll ( final Stream < ? extends T > stream ) { final Set < T > set = stream . collect ( Collectors . toSet ( ) ) ; return filter ( null ) ; }
retain only the supplied elements in the returned filterable
