Move the begin point of this connector to coordinates of the
        connection point of *shape* specified by *cxn_pt_idx*.

def _move_begin_to_cxn(self, shape, cxn_pt_idx):
        """
        Move the begin point of this connector to coordinates of the
        connection point of *shape* specified by *cxn_pt_idx*.
        """
        x, y, cx, cy = shape.left, shape.top, shape.width, shape.height
        self.begin_x, self.begin_y = {
            0: (int(x + cx/2), y),
            1: (x, int(y + cy/2)),
            2: (int(x + cx/2), y + cy),
            3: (x + cx, int(y + cy/2)),
        }[cxn_pt_idx]