you can't erase parts of the drawn lines, but there are other
ways to achieve erasing:
A) If you have a constant background color (e.g. when drawing
on a white canvas), you could just draw over the drawn content with
the background color, this will look like the content was erased.
B) You store the drawn points (= the mouse point in every
mouseMove) in an array, and split this array into segments when
erasing parts (=points) of the line. Then delete all drawn content
(canvas_mc.clear()), and redraw the remaining points.