group law of Elliptic curve with taking two point at infinity

Question

We often describe group law on elliptic curve, like 'taking two points from $E$, and then line it rhrough infinity, and the line intersects with another new point, and line through the new point with infinity. In particular, if we take two points at the same point of affine chart of $E$, the first line is the tangent at the overlapping first two points. But what happen when we take first two points of the line at infinity? How can I describe the group law on this case? In particular, what is the first line of the overlapping first two points at infinity?