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_preserves_out :
forall A B C A' B' C' M,
Out A B C ->
Midpoint M A A' ->
Midpoint M B B' ->
Midpoint M C C' ->
Out A' B' C'.