Drawing board ,draw straight line

i heave learnt how to create a drawing board in flash mx 2004 for use to draw line from this thread http://www.kirupa.com/developer/actionscript/drawingboard.htm
but i need some help here

Question1:ie. i draw 3 line means i am in step 3 , how can i undo to step 2 ?how to create actionscript for undo?

Question2: how to erase for example the line i drawn out in this drawing board during test scene or test movie?

Question3:how can i specify the range/area for the drawing board as i have no intention for it to cover full scale ?