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

Lemma suma_assoc_1 : forall A B C D E F G H I K L M A' B' C' D' E' F',
SAMS A B C D E F -> SAMS D E F G H I ->
SumA A B C D E F A' B' C' -> SumA D E F G H I D' E' F' ->
SumA A' B' C' G H I K L M -> SumA A B C D' E' F' K L M.