(** Commutativity of the sum. *)

(* Specifications: *)
Lemma suma_sym : forall A B C D E F G H I, SumA A B C D E F G H I -> SumA D E F A B C G H I.
(** CongA preserves SumA. *)
Lemma conga3_suma__suma : forall A B C D E F G H I A' B' C' D' E' F' G' H' I',
SumA A B C D E F G H I ->
CongA A B C A' B' C' ->
CongA D E F D' E' F' ->
CongA G H I G' H' I' ->
SumA A' B' C' D' E' F' G' H' I'.