(** Lemma 16.2 in dimension 2. *)

(** As we are in dimension 2, we skip 16.3 which is only needed to prove 16.4 in dimension n. *)
(** Lemma 16.4 in dimension 2. *)
Lemma exists_grid : exists O E E' S U1 U2, ~ Col O E E' /\ Cs O E S U1 U2.