(** * Proposition 15 If two straight lines cut one another, they make the vertical angles equal to one another. *)
(** # <div style="width:748px;height:397px;display:block" id="applet_container15"></div> # **)

Lemma prop_15 : forall A B C D E, Bet A E B -> A <> E -> B <> E ->
Bet C E D -> C <> E -> D <> E ->
CongA A E C B E D.