Section INCIDENCE.
(* I1 : For any two distinct points A, B, there exists a unique line L containing A, B. *)
(* I2 : Every line contains at least two points. *)
(* I3 : There exist three noncollinear points (that is, three points not all contained in a single line). *)

Lemma I3 : exists A : Point, exists B : Point, exists C : Point, forall F : Figure, forall D : Line F, ~(F A /\ F B /\ F C).