(** This is SSA congruence with a right angle *)

Lemma l11_61 : forall A B C A' B' C',
A <> A' -> A <> B -> A <> C ->
Coplanar A A' B B' -> Per B A A' -> Per B' A' A ->
Coplanar A A' C C' -> Per C A A' ->
Per B A C -> Per B' A' C'.