symmetry  in |- *.
Geometry.
Qed.

(* Specifications: *)
Lemma trivial_col2 : forall A B : Point, 0 = S A B B.
intros.
Lemma trivial_col3 : forall A B : Point, 0 = S A B A.
intros.
Lemma A1a : forall A B : Point, A ** B = - B ** A.
Lemma degenerated_ratio : forall A B C D,
A=B -> C<>D -> A**B/C**D=0.