Hint Resolve perp_per_1 perp_per_2 perp_col perp_perp_in perp_in_perp : perp.
Section T8_5.
Context `{TnEQD:Tarski_neutral_dimensionless_with_decidable_point_equality}.

Lemma perp_proj : forall A B C D, Perp A B C D -> ~Col A C D -> exists X, Col A B X /\ Perp A X C D.