public function testSubtract()
{
$bn = new BigNumber(2147483647);
$this->assertSame($bn, $bn->subtract('9223372036854775808'));
$this->assertSame('-9223372034707292161', $bn->getValue());
$bn->setScale(3);
$this->assertSame('-9223372034707292163.250', $bn->subtract(2.25)->getValue());
}