Section T7_2.
Context {Tn:Tarski_neutral_dimensionless}.
Context {TnEQD:Tarski_neutral_dimensionless_with_decidable_point_equality Tn}.
(** This is Krippen lemma , proved by Gupta in its PhD in 1965 as Theorem 3.45 *)

Lemma midpoint_def : forall A B C, Bet A B C -> Cong A B B C -> Midpoint B A C.