function testChangeScoreFor()
 {
     $leaderboard = new Leaderboard('leaderboard');
     $leaderboard->changeScoreFor('member_1', 5);
     $this->assertEquals(5, $leaderboard->scoreFor('member_1'));
     $leaderboard->changeScoreFor('member_1', 5);
     $this->assertEquals(10, $leaderboard->scoreFor('member_1'));
     $leaderboard->changeScoreFor('member_1', -5);
     $this->assertEquals(5, $leaderboard->scoreFor('member_1'));
 }