Section T15.
Context `{T2D:Tarski_2D}.
Context `{TE:@Tarski_euclidean Tn TnEQD}.
(** Lemma 15.2 *)
(** Cong corresponds to length equality.*)
(** Le corresponds to length inequality.*)
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Lemma sum_preserves_ltp : forall O E E' A B C AC BC, LtP O E E' A B -> Sum O E E' A C AC -> Sum O E E' B C BC -> LtP O E E' AC BC.