Besides helping out in scripting, I have been slow for my own job because of boredom ascribed to continuous trials but with no results. It was confusing and it had to be. I was doing the right things but at wrong places. 😛
So I kept line again aside and starting hunting the undo/redo implementation for circle and ellipse. Though logic was same but even trivial things like displaying correct information in the undo stack seemed of much significance to us. This was tricky since I was doing it all in the wrong place. But it worked finally.
Then the bulb glowed again. And I did undo/redo for line without constructor over-riding now. It involved so much less effort today. And that feeling when your commit reduces the lines of code and does the same work correctly. So finally I am done with my undo/redo task. It is over. 😀