@Test
 public void testPercentageOf() {
   MutableMoney money = MutableMoney.ONE;
   assertEquals(0.1, money.percentageOf(MutableMoney.TEN), 0.0001);
   money = MutableMoney.of(774, 27);
   assertEquals(0.0042, money.percentageOf(MutableMoney.of(182737, 9)), 0.0001);
 }