public function testIntersectK()
{
$collection1 = new Collection(['a', 'b', 'e']);
$collection2 = new Collection(['a', 'e']);
$collection3 = new Collection(['a', 'c', 'e']);
$result = $collection1->intersectK($collection2, $collection3);
$this->assertNotSame($collection1, $result);
$this->assertInstanceOf(get_class($collection1), $result);
$this->assertSame([0 => 'a', 1 => 'b'], (array) $result);
}
