unfold Cong in *.
induction H.
left; intuition.
apply congH_sym.
assumption.
assumption.
right.
tauto.
Qed.

Lemma soustraction_betH :
forall A B C A' B' C' : Point,
BetH A B C ->
BetH A' B' C' ->
CongH A B A' B' -> CongH A C A' C' -> CongH B C B' C'.