intros.
unfold Per in *.
DecompExAnd H C'.
suppose (Cong A' C A' C').
assert (C=C').
eapply l4_19.
2:apply H4.
2:apply H.
assumption.
subst C'.
eapply l7_3.
assumption.
DecompExAnd H0 C''.
assert (Cong C C C' C'').
eapply l7_13.
apply l7_2.
apply H3.
apply l7_2.
apply H2.
treat_equalities.
auto.
Qed.
Ltac let_symmetric C P A :=
let id1:=fresh in (assert (id1:(exists A', is_midpoint P A A'));
[apply symmetric_point_construction|DecompEx id1 C]).

Lemma l8_7 : forall A B C, Per A B C -> Per A C B -> B=C.
Hint Resolve l8_7 : Perp.