public function testFilter()
{
$seq = new Sequence([1, 2, 3]);
$newSeq = $seq->filter(function ($n) {
return $n === 2;
});
$this->assertNotSame($newSeq, $seq);
$this->assertCount(3, $seq);
$this->assertCount(1, $newSeq);
$this->assertSame(2, $newSeq->get(0));
}