Ltac ang_instance1 a A B C :=
assert(tempo_ang:= ang_const a A B);
match goal with
|H: Q_CongA a |-  _ => assert(tempo_H:= H);apply tempo_ang in tempo_H; ex_elim tempo_H C
end;
clear tempo_ang.
Section Angles_4.
Context `{TnEQD:Tarski_neutral_dimensionless_with_decidable_point_equality}.

Lemma ang_sym : forall a A B C, Q_CongA a -> a A B C -> a C B A.