(** Lemma 16.2 in dimension 2. *)

Lemma bet_betCood_aux : forall O E S U1 U2 A AX AY B BX BY C CX CY,
Cd O E S U1 U2 A AX AY -> Cd O E S U1 U2 B BX BY -> Cd O E S U1 U2 C CX CY ->
Bet A B C ->
Bet AX BX CX.