code: public static function concat(IteratorAggregate $a, IteratorAggregate $b) : IteratorAggregate
      {
          return self::from(Iterators::concat(Iterators::from($a->getIterator()), Iterators::from($b->getIterator())));
      }
docs: Combines two iterables into a single iterable.
  The returned iterable has an iterator that traverses the elements in a,
  followed by the elements in b. The source iterators are not polled until necessary.
  
  @param IteratorAggregate $a
  @param IteratorAggregate $b
  @return IteratorAggregate