def intersects(self, line2):
        # oversimplification?
        if self.dir == line2.dir:
            return []
        if self.dir == "X":
            assert line2.p1.x == line2.p2.x
            if self.p1.x < line2.p1.x and self.p2.x > line2.p1.x:
                if line2.p1.y < self.p1.y and line2.p2.y > self.p1.y:
                    return [Pt(line2.p1.x, self.p1.y)]
                else:
                    return []
            else:
                return []
        elif self.dir == "Y":
            assert line2.dir == "X"
            return line2.intersects(self)
        else:
            raise RuntimeError("Hm, F.")