(PECL ds >= 1.0.0)
Ds\Sequence::first — Returns the first value in the sequence.
此函数没有参数。
The first value in the sequence.
UnderflowException if empty.
Example #1 Ds\Sequence::first() example
<?php
$sequence = new \Ds\Vector([1, 2, 3]);
var_dump($sequence->first());
?>
以上例程的输出类似于：
int(1)