Section Concyclic.
Context `{TE:Tarski_euclidean}.
Definition Concyclic A B C D := Coplanar A B C D /\ exists O, Cong O A O B /\ Cong O A O C /\ Cong O A O D.

Lemma concyclic_perm_18 : forall A B C D,
Concyclic A B C D -> Concyclic D A B C.