End Grid.

(** Lemma 14.25 *)
(** Left distributivity of product over sum.*)
Lemma distr_l : forall O E E' A B C D AB AC AD,
Sum O E E' B C D -> Prod O E E' A B AB -> Prod O E E' A C AC ->
(Prod O E E' A D AD -> Sum O E E' AB AC AD).