code: public static function concat(Iterator $a, Iterator $b) : Iterator
      {
          return self::concatIterators(new ArrayIterator([$a, $b]));
      }
docs: Combines two iterators into a single iterator. The returned iterator iterates across the elements in a,
  followed by the elements in b. The source iterators are not polled until necessary.
  
  @param Iterator $a
  @param Iterator $b
  @return Iterator