assert (l.getDirection() == getDirection()) &&
assert ((l.getDirection() == getDirection()) || (l.x1 == l.x2 && l.y1 == l.y2)) &&
    public void prepend(IntLine l) {	assert (l.getDirection() == getDirection()) &&	    (l.x2 == x1 && l.y2 == y1);	x1 = l.x1;	y1 = l.y1;    }