src/precore/util/Iterators.php
public static function concat(Iterator $a, Iterator $b) : Iterator
    {
        return self::concatIterators(new ArrayIterator([$a, $b]));
    }
public static function concat(Iterator $a, Iterator $b) : Iterator
    {
        return self::concatIterators(new ArrayIterator([$a, $b]));
    }
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