Erase using brush in GLPaint

Question

As part of modifying the GLPaint, I am trying to add erase functionality where user could select an eraser button and erase the painted area just as painting.

I am trying have a conditional statement within "renderLineFromPoint:(CGPoint)start toPoint:(CGPoint)end" method so that I could check whether the stroke is for painting or erasing.

For erasing I do not know how to make use of the "start" and "end" parameters for erasing. Is there any method call in OpenGL like glClear() that accepts these two parameter and does erase?

Any pointer will be very helpful. Thank you.