Proof with finish.
intros A A1 B B1 C C1 O HNC HPerp1 HPerp2 HPerp3 HC1 HC2.
assert (HT := HNC).
apply construct_triangle in HT.
destruct HT as [D [E [F HT]]].
spliter.
assert (Midpoint A E F) by (apply diff_not_col_col_par4_mid with B C; finish).
assert (Midpoint B D F) by (apply diff_not_col_col_par4_mid with A C; finish).
assert (Midpoint C D E) by (apply diff_not_col_col_par4_mid with A B; finish).
assert_diffs.
elim (eq_dec_points A O); intro.
treat_equalities; apply col_permutation_4; apply cop_perp2__col with A B...
apply perp_right_comm; apply perp_col1 with B1...
elim (eq_dec_points B O); intro.
treat_equalities; apply col_permutation_4; apply cop_perp2__col with A B...
apply perp_col1 with A1...
elim (eq_dec_points C O); intro.
subst; Col.
assert (Perp_bisect A O E F) by (apply altitude_is_perp_bisect with B C A1; finish).
assert (Perp_bisect B O D F) by (apply altitude_is_perp_bisect with A C B1; finish).
assert (Perp O C D E).
apply circumcenter_intersect with F A B; finish.
apply perp_bisect_sym_1; assumption.
apply perp_bisect_sym_1; assumption.
assert (Perp C1 C D E).
apply perp_sym; apply cop_par_perp__perp with A B...
apply par_symmetry; apply par_col_par_2 with C...
apply col_permutation_2; apply cop_perp2__col with D E; Perp.
apply coplanar_pseudo_trans with A B C.
assumption.
Cop.
apply col_cop__cop with D; Col; Cop.
Cop.
apply coplanar_perm_2, col_cop__cop with B1; Col; Cop.
Qed.

Lemma is_orthocenter_perm_3 : forall A B C G,
is_orthocenter G A B C -> is_orthocenter G B C A.