Require Export GeoCoq.Tarski_dev.Definitions.
Require Export GeoCoq.Tactics.finish.
Ltac prolong A B x C D :=
assert (sg:= segment_construction A B C D);
ex_and sg x.
Section T1_1.
Context `{Tn:Tarski_neutral_dimensionless}.

Lemma cong_left_commutativity : forall A B C D,
Cong A B C D -> Cong B A C D.