Section T13.
Context `{TE:Tarski_euclidean}.

Lemma l12_16 : forall A1 A2 B1 B2 C1 C2 X,
Par A1 A2 B1 B2 -> Coplanar B1 B2 C1 C2 -> Inter A1 A2 C1 C2 X -> exists Y, Inter B1 B2 C1 C2 Y.