Section T14_sum.
Context `{T2D:Tarski_2D}.
Context `{TE:@Tarski_euclidean Tn TnEQD}.
(* a inclure dans project.v *)

Lemma project_col_project : forall A B C P P' X Y,
A <> C -> Col A B C ->
Proj P P' A B X Y ->
Proj P P' A C X Y.