(** Unicity of the difference of angles. *)

Lemma sams2_suma2__conga456 : forall A B C D E F 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 D E F D' E' F'.