Definition AM_CongAL A B C D E F := Py A B C * twice_signed_area D E F =F= Py D E F * twice_signed_area A B C.
(** This shows that we have a notion of congruence of line angles, every angle is congruent to its supplement. *)

Lemma supplement_AM_CongA : forall A B M C, Midpoint M A B -> AM_CongAL  A M C B M C.