Section Projections.
Context `{T2D:Tarski_2D}.
Context `{TE:@Tarski_euclidean Tn TnEQD}.
intros.
unfold Proj in H0.
spliter.
induction H4; tauto.
Qed.
intros.
unfold Proj in *.
spliter.
repeat split; Col.
Qed.
intros.
unfold EqV in H.
induction H.
apply plg_cong.
apply plg_permut.
apply plg_comm2.
assumption.
spliter.
subst B.
subst D.
Cong.
Qed.
intros.
unfold Projp.
split.
apply l8_14_2_1a in H.
apply perp_not_eq_1 in H.
assumption.
left.
split.
apply perp_in_col in H.
spliter.
assumption.
apply l8_14_2_1a in H.
assumption.
Qed.
intros.
unfold Projp in H.
spliter.
induction H0.
spliter.
induction(eq_dec_points P' A).
subst P'.
right.
apply perp_not_eq_1 in H1.
assumption.
left.
assumption.
spliter.
subst P'.
induction(eq_dec_points P A).
subst P.
right.
assumption.
left.
assumption.
Qed.
intros.
unfold Projp in *.
spliter.
induction H1; induction H2.
spliter.
apply (l8_18_uniqueness A B P); Col.
apply perp_not_col2 in H4.
induction H4.
assumption.
contradiction.
spliter.
subst P'.
apply perp_not_col2 in H3.
induction H3; contradiction.
spliter.
subst Q'.
apply perp_not_col2 in H4.
induction H4; contradiction.
spliter.
subst P'.
subst Q'.
auto.
Qed.

Lemma projp_projp_perp : forall P P1 P2 Q1 Q2,
P1 <> P2 -> Projp P1 P Q1 Q2 -> Projp P2 P Q1 Q2 -> Perp P1 P2 Q1 Q2.