Section SECANT_LINES_PROPERTIES.

Lemma PaschABC  : forall A B C M N,
Clockwise A B C ->
Between A M B  ->
~Collinear A M N ->
~Collinear B M N ->
~Collinear C M N ->
(exists P : Point, Collinear M N P /\ Between A P C) \/
(exists P : Point, Collinear M N P /\ Between B P C).