public void ItAddsCorrectly(double before, double amount, double after)
{
var initial = new FahrenheitTemperature(before);
var result = initial.Add(amount);
Assert.That(initial, Is.Not.SameAs(result));
Assert.That(initial.Value, Is.EqualTo(before));
Assert.That(result.Value, Is.EqualTo(after));
}