public void TaxiDistanceWorks(int x1, int y1, int x2, int y2, int distance)
{
var p = new XYPoint(x1, y1);
var q = new XYPoint(x2, y2);
Assert.That(p.DistanceFrom(q), Is.EqualTo(distance));
Assert.That(q.DistanceFrom(p), Is.EqualTo(distance));
}