Theorem th6_256_1 : forall (A B C D P Q R S : Point) (r1 r2 : F), on_parallel_d D A B C 1 -> on_line_d S D A r2 -> on_line_d P A B r1 -> on_line_d R C D r1 -> on_line_d Q B C r2 -> P <> S -> parallel Q R P S.