(* shows that we need A<>B in elim_py_on_line_d_middle Lemma essai : forall A P Q Y : Point, forall r:F, r<> 0 -> on_line_d Y P Q r -> False. *)

Theorem elim_py_inter_ll_left_right :
forall B P Q U V Y : Point,
inter_ll Y P Q U V ->
Py Y B Y = 1 / S4 P U Q V *
(S P U V * (1 / S4 P U Q V * (S P U V * Py Q B Q + S Q V U * Py Q B P)) +
S Q V U * (1 / S4 P U Q V * (S P U V * Py P B Q + S Q V U * Py P B P))).