public function testRemoveClassmate()
 {
     $john = new DBStudent();
     $jean = new DBStudent();
     $this->assertEquals(0, $john->countClassmates());
     $this->assertEquals(0, $jean->countClassmates());
     $john->addClassmate($jean);
     $this->assertEquals(1, $john->countClassmates());
     $this->assertEquals(1, $jean->countClassmates());
     $john->removeClassmate($jean);
     $this->assertEquals(0, $john->countClassmates());
     $this->assertEquals(0, $jean->countClassmates());
 }