Section T11_2.
Context `{TnEQD:Tarski_neutral_dimensionless_with_decidable_point_equality}.
(** This is exterior angle theorem *)
(** In an isosceles triangle the two base angles are equal. This is Euclid: Book 1, Proposition 5. *)
(** This is Euclid: Book 1, Proposition 18 *)

Lemma lta_not_conga : forall A B C D E F, LtA A B C D E F -> ~ CongA A B C D E F.