unfold EqV in H.
induction H.
apply plg_cong.
apply plg_permut.
apply plg_comm2.
assumption.
spliter.
subst B.
subst D.
Cong.
Qed.

Lemma perp_projp : forall P P' A B, Perp_at P' A B P P' -> Projp P P' A B.
intros.