public void TestDefaultArgs(IResultSink res)
{
Peter p = new Peter();
p.SetSink(res);
p.TellYourAge(); // expected -1, result: -1
p.DoSomething(); // expected -1, result: 0
Fred f = new Fred();
f.SetSink(res);
f.TellYourAge(1); // expected 1, result: 1
f.DoSomething(); // expected 1, result: 1
}