End Grid.

(** Lemma 14.22 *)
(** Parallel projection on the second axis preserves products. *)
Lemma proj_preserves_prod : forall O E E' A B C A' B' C',
Prod O E E' A B C -> Ar1 O E' A' B' C' ->
Pj E E' A A' -> Pj E E' B B' -> Pj E E' C C' ->
Prod O E' E A' B' C'.