inters

(inters p1 p2 p3 p4 [on-segments])

Intersection point of line p1-p2 with line p3-p4, or nil if they are parallel. If the fifth argument is nil, the lines are extended; if it is missing or not nil, the point must lie on both segments.

(inters '(0 0) '(10 10) '(0 10) '(10 0))   ; (5.0 5.0 0.0)