public void CanOverrideChangeCompletion()
        {
            // arrange: change, then "don't" complete, then change
            history.CurrentChange.Add(1);
            using (history.ContinueCurrentTransaction()) history.ChangeCompleted();
            history.CurrentChange.Add(2);

            // act: undo
            history.Undo.Execute();

            // assert: that was the only thing in the undo stack
            Assert.False(history.Undo.CanExecute(null));
        }