unfold eqo.
repeat split.
eapply one_side_not_col123.
apply H.
apply one_side_symmetry in H.
eapply one_side_not_col123.
apply H.
intros.
apply l7_3 in H6.
subst M.
induction(eq_dec_points C C1).
subst C1.
left.
apply midpoint_bet.
assumption.
assert(A <> C).
apply perp_not_eq_2 in H0.
auto.
assert(A <> C1).
apply perp_not_eq_2 in H2.
auto.
assert(A <> C /\ A <> C1 /\ C <> C1).
repeat split; auto.
assert(Col A C C1).
eapply perp2__col.
apply perp_comm.
apply perp_sym.
apply H0.
apply perp_comm.
Perp.
assert(HH:=ex_col A C C1 H12 H13).
ex_and HH D.
assert(Perp A B D C).
apply perp_sym.
apply perp_left_comm.
eapply perp_col.
auto.
apply perp_sym.
apply H0.
Col.
assert(~Col A B X).
unfold OS in H.
ex_and H T.
unfold TS in H.
spliter.
intro.
apply H.
Col.
assert(~Col A B Y).
unfold OS in H.
ex_and H T.
unfold TS in H22.
spliter.
intro.
apply H22.
Col.
assert(OS A B X C).
eapply (per_one_side A B D); auto.
apply  col_permutation_3.
apply H14.
Col.
assumption.
eapply per_col.
2: apply H1.
auto.
Col.
assert(OS A B Y C1).
eapply (per_one_side A B D); auto.
apply col_permutation_3.
apply H14.
apply col_permutation_2.
eapply (col_transitivity_1 _ A);Col.
assumption.
eapply per_col.
2: apply H3.
auto.
apply col_permutation_2.
eapply (col_transitivity_1 _ C);
Col.
assert(OS A B C C1).
eapply one_side_transitivity.
apply one_side_symmetry.
apply H23.
eapply one_side_transitivity.
apply H.
assumption.
assert(A <> B).
apply perp_not_eq_1 in H0.
assumption.
assert(~ Col C1 A B).
unfold OS in H24.
ex_and H24 T.
unfold TS in H27.
spliter.
assumption.
assert(TS A B C1 C').
unfold TS.
repeat split; auto.
intro.
unfold Midpoint in H8.
spliter.
assert(C'=A).
eapply l6_21.
apply not_col_permutation_1.
apply H27.
4: apply bet_col.
4: apply H8.
auto.
Col.
Col.
Col.
subst C'.
apply cong_identity in H29.
subst C1.
tauto.
exists A.
unfold Midpoint in H8.
spliter.
split; auto.
Col.
assert(TS A B C C').
eapply l9_8_2.
apply H28.
apply one_side_symmetry.
assumption.
assert(Col A C C').
eapply (col_transitivity_1 _ C1).
assumption.
Col.
unfold Midpoint in H8.
spliter.
apply bet_col in H8.
Col.
unfold TS in H29.
spliter.
ex_and H32 AA.
assert(AA=A).
eapply l6_21.
apply not_col_permutation_1.
apply H31.
4: apply col_permutation_2.
4: apply bet_col.
4: apply H33.
intro.
subst C'.
apply between_identity in H33.
subst AA.
apply H31.
assumption.
Col.
Col.
Col.
subst AA.
left.
assumption.
Qed.

Lemma ex_col1 : forall A B C, A <> B -> Col A B C -> exists D, Col A B D /\ A <> D /\ B <> D /\ C <> D.
intros.