intros.
assert(HH:= vector_construction C D B).
ex_and HH F.
exists A.
exists F.
unfold SumV.
intros.
assert(D' = F).
apply (vector_construction_uniqueness C D B); auto.
subst D'.
apply eqv_refl.
Qed.

(* Specifications: *)
Lemma sum_uniqueness :
forall A B C D E F E' F',
SumV A B C D E F ->
SumV A B C D E' F' ->
EqV E F E' F'.
Lemma same_dir_refl : forall A B, Same_dir A B A B.
intros.