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. *)

Lemma l11_44_1_a : forall A B C, A <> B -> A <> C -> Cong B A B C -> CongA B A C B C A.