Definition TCong A B C D := Cong A B C D \/ (A=B /\ C=D).

(* Specifications: *)
Lemma cong_sym : forall A B C D, Cong A B C D -> Cong C D A B.
Lemma cong_eq : forall A B C, Cong B C A A -> B=C.
Lemma nullsegment2 : forall A B, TCong A A B B.
Lemma Tcong_reflexive: forall A B, TCong A B A B.
Lemma Tcong_symmetric: forall A B C D, TCong A B C D -> TCong C D A B.
Lemma nullsegment1 : forall A B C, TCong A A B C -> B=C.
Lemma TCong_neq_Cong : forall A B C D, A<>B -> TCong A B C D -> Cong A B C D.
Lemma lemma_congruenceflip :
forall A B C D,
Cong A B C D ->
Cong B A D C.