intros x y H'.
apply thm4_1a.
apply sym_DiLn; auto.
Qed.

(* Specifications: *)
Theorem thm4_3a :
forall (x : Segment) (c : Point), Apart c (ln x) -> DiPt c (origin x).
Theorem thm4_3b :
forall (x : Segment) (c : Point), Apart c (ln x) -> DiPt c (extremity x).