(** This is Krippen lemma , proved by Gupta in its PhD in 1965 as Theorem 3.45 *)

Lemma l7_22 : forall A1 A2 B1 B2 C M1 M2,
Bet A1 C A2 -> Bet B1 C B2 ->
Cong C A1 C B1 -> Cong C A2 C B2 ->
Midpoint M1 A1 B1 -> Midpoint M2 A2 B2 ->
Bet M1 C M2.