Section SUM_ANGLES_PROPERTIES.

Lemma SumAnglesCorollary : forall A B C D E A' B' C' : Point,
Clockwise A' B' C' ->
Clockwise A B C ->
Clockwise A C D ->
Angle B A C = Angle A' B' C' ->
Angle C A D = Angle B' A' C' ->
Between B A E ->
Angle D A E = Angle A' C' B'.