public void NonNegative_NullablePreconditionMethod_ThrowsIfNegative()
        {
            var demo = new NonNegativeConsumer();

            Assert.Throws <PreconditionException>(() =>
                                                  demo.PreconditionNullableMethod(-1));
        }
        public void NonNegative_NullablePreconditionMethod_DoesNotThrowIfNull()
        {
            var demo = new NonNegativeConsumer();

            Assert.DoesNotThrow(() =>
                                demo.PreconditionNullableMethod(null));
        }