(* Lemma test_on_line_d_13 : forall Y P Q A C D lambda, on_line_d Y P Q lambda ->  Y<>A -> parallel C D Y A -> False -> C**D/Y**A = C**D. *)
(** Test inter_ll in a ratio *)
(** Test cases where Y occurs both in denominator and numerator *)

Lemma test_on_foot_2 : forall A C D Y R P Q,
on_foot Y R P Q ->
False ->
C <> D ->
parallel Y A C D ->
Y**A/C**D = - - (Y**A/C**D).