php 技术 yield 问题?报错

起因 

http://www.oschina.net/translate/cooperative-multitasking-using-coroutines-in-php

没看懂

<?php

var_dump($gen->send('ret1')); //输出是 yield2  不理解为什么

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function nums() {
    for ($i = 0; $i < 5; ++$i) {
                //get a value from the caller
        $cmd = (yield $i);
        
        if($cmd == 'stop')
            return;//exit the function
        }  
}

$gen = nums();

var_dump( $gen->current());// 0 
$gen->next();
var_dump( $gen->current());// 1
$gen->next();

$gen->rewind(); // 我以为这里 会将指针 回归到 0 可是报错

PHP Fatal error:  Uncaught exception 'Exception' with message 'Cannot rewind a generator that was already run' in /var/www/html/learn.php:28
Stack trace:
#0 /var/www/html/ams/learn.php(28): Generator->rewind()
#1 {main}
  thrown in /var/www/html/ams/learn.php on line 28

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		var_dump( $gen->current());// 我以为会输出 0 <br>
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请老师教我