intros.
assert (- -a=-0).
rewrite H...
assert (- -a=a)...
assert (-0=0)...
Qed.
Hint Resolve field_prop1 : Geom.
(** Some propositions about Col *)

Theorem col_1 : forall A B C : Point, Col A B C -> Col B A C.
Proof with uniformize_signed_areas;Geometry.