'Set or disable undobuffer.
Argument:
size -- an integer or None
If size is an integer an empty undobuffer of given size is installed.
Size gives the maximum number of turtle-actions that can be undone
by the undo() function.
If size is None, no undobuffer is present.
Example (for a Turtle instance named turtle):
>>> turtle.setundobuffer(42)'

def setundobuffer(self, size):

    if (size is None):
        self.undobuffer = None
    else:
        self.undobuffer = Tbuffer(size)
