Hint Resolve not_cong_2134 not_cong_1243 not_cong_2143
not_cong_3412 not_cong_4312 not_cong_3421 not_cong_4321 : cong.
Section T1_3.
Context `{Tn:Tarski_neutral_dimensionless}.

Lemma cong_3_sym : forall A B C A' B' C',
Cong_3 A B C A' B' C' -> Cong_3 A' B' C' A B C.