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_in_per_4 :
forall A B C D X,
Perp_at X A B C D ->
Per B X D.