cbOne (double a, double b)

	std::cout << "invoke cbOne a=" << a << ", b=" << b << std::endl;

	int cbTwo (double a) {

		std::cout << "invoke cbTwo a=" << a << std::endl;

	// build callback instance which points to cbOne function

	one = makeCallback (&cbOne);

	assert (!one.isNull ());

	// invoke cbOne function through callback instance

	double retOne;

	retOne = one (10.0, 20.0);

	// build callback instance which points to MyCb::cbTwo

	two = makeCallback (&MyCb::cbTwo, &cb);

	assert (!two.isNull ());

	// invoke MyCb::cbTwo through callback instance

	int retTwo;

	retTwo = two (10.0);	

	two = makeNullCallback<int, double> ();

	//int retTwoNull = two (20.0);

	assert (two.isNull ());