public function testSubtraction()
{
$m1 = Money::fromAmount('100', Currency::fromCode('EUR'));
$m2 = Money::fromAmount('200', Currency::fromCode('EUR'));
$diff = $m1->subtract($m2);
$expected = Money::fromAmount('-100', Currency::fromCode('EUR'));
$this->assertTrue($diff->equals($expected));
// Should return a new instance
$this->assertNotSame($diff, $m1);
$this->assertNotSame($diff, $m2);
}