Section Grid.
Variable O E E' : Tpoint.
Variable grid_ok : ~ Col O E E'.
End Grid.

(** Lemma 14.23 *)
(** Product is associative.*)
Lemma prod_assoc1 : forall O E E' A B C AB BC ABC,
Prod O E E' A B AB -> Prod O E E' B C BC ->
(Prod O E E' A BC ABC -> Prod O E E' AB C ABC).