intros.
cases_equality A B.
left;auto.
right.
assert (Per C B C).
eapply l8_3...
eauto with Col.
Perp.
Qed.

Lemma l8_10 : forall A B C A' B' C',
Per A B C -> Cong_3 A B C A' B' C' -> Per A' B' C'.