public function testIndexContains()
 {
     $this->collection->add('Hye');
     $this->collection->add(34);
     $this->collection->add(true);
     $this->assertEquals($this->collection->indexOf(true), 2);
     $this->assertEquals($this->collection->indexOf(34), 1);
     $this->assertTrue($this->collection->contains(34));
 }