Section Grid.
Variable O E E' : Tpoint.
Variable grid_ok : ~ Col O E E'.
End Grid.
(** We omit lemma 14.26 which states that we have a division ring. *)

(** Lemma 14.27. *)
(** Sum and product are preserved by parallel projection on a different x-axis.*)
(** This lemma is used to prove that there is an isomorphism between number lines.*)
Lemma prod_1_l_eq : forall O E E' A B, Prod O E E' A B B -> A = E \/ B = O.