public function testSumTimes()
 {
     $fiveBucks = Money::dollar(5);
     $tenFrancs = Money::franc(10);
     $bank = new Bank();
     $bank->addRate('CHF', 'USD', 2);
     $step1 = new Sum($fiveBucks, $tenFrancs);
     $sum = $step1->times(2);
     $result = $bank->reduce($sum, 'USD');
     $this->assertEquals(Money::dollar(20), $result);
 }