(** Cong preserves SumS *)

(* Specifications: *)
Lemma cong3_sums__sums : forall A B C D E F A' B' C' D' E' F',
Cong A B A' B' -> Cong C D C' D' -> Cong E F E' F' -> SumS A B C D E F ->
SumS A' B' C' D' E' F'.