Definition orp_witnessed: context -> Prop :=
  fun Phi => forall x y, Phi (orp x y) -> Phi x \/ Phi y.Definition context_orp_captured (P: context -> Prop): Prop :=
  forall Phi Psi, P (context_orp Phi Psi) -> P Phi \/ P Psi.

Lemma DCS_multi_and_iff: forall (Phi: context), derivable_closed Phi -> (forall xs: list expr, Phi (multi_and xs) <-> Forall Phi xs).