(** Unicity of the difference on the left. *)

Lemma sams2_suma2__conga123 : forall A B C A' B' C' D E F G H I,
SAMS A B C D E F -> SAMS A' B' C' D E F ->
SumA 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'.