code: public static function filterBy(Iterator $unfiltered, string $className) : Iterator
      {
          return self::filter($unfiltered, Predicates::instance($className));
      }
docs: Returns all instances of class type in unfiltered.
  The returned iterator has elements whose class is type or a subclass of type.
  
  @param Iterator $unfiltered
  @param string $className
  @return Iterator