这个迭代器能陆续遍历几个迭代器 ``` AppendIterator extends IteratorIterator implements OuterIterator { /* 方法 */ public __construct ( void ) public append ( Iterator $iterator ) : void public current ( void ) : mixed public getArrayIterator ( void ) : ArrayIterator public getInnerIterator ( void ) : Iterator public getIteratorIndex ( void ) : int public key ( void ) : scalar public next ( void ) : void public rewind ( void ) : void public valid ( void ) : bool /* 继承的方法 */ public IteratorIterator::__construct ( Traversable $iterator ) public IteratorIterator::current ( void ) : mixed public IteratorIterator::getInnerIterator ( void ) : Traversable public IteratorIterator::key ( void ) : mixed public IteratorIterator::next ( void ) : void public IteratorIterator::rewind ( void ) : void public IteratorIterator::valid ( void ) : bool } ```