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))).