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.*)

(** Known as Euklid *)
Lemma l15_7 : forall O E E' A B C H AB AC AH AC2,
O<>E -> Per A C B -> Perp_at H C H A B ->
Length O E E' A B AB -> Length O E E' A C AC -> Length O E E' A H AH ->
(Prod O E E' AC AC AC2 <-> Prod O E E' AB AH AC2).