Section T16.
Context `{T2D:Tarski_2D}.
Context `{TE:@Tarski_euclidean Tn TnEQD}.
(** Lemma 16.2 in dimension 2. *)
(** As we are in dimension 2, we skip 16.3 which is only needed to prove 16.4 in dimension n. *)

Lemma length_eq_or_opp : forall O E E' A B L1 L2,
Length O E E' A B L1 -> Diff O E E' A B L2 -> L1 = L2 \/ Opp O E E' L1 L2.