proposition: Lemma grid_exchange_axes : forall O E S U1 U2, Cs O E S U1 U2 -> Cs O E S U2 U1.
proof: Proof. intros O E S U1 U2 HCs. destruct HCs as [HDiff [HCong1 [HCong2 HPer]]]. repeat (split; Perp). Qed.
imports: GeoCoq.Tarski_dev.Ch15_lengths