Definition all_iff (P0 : Prop) (Ps : seq Prop) : Prop :=
  (fix aux (P : Prop) (Qs : seq Prop) : Prop :=
      if Qs is Q :: Qs then all_iff_and (P -> Q) (aux Q Qs)
      else P -> P0 : Prop) P0 Ps.

Lemma all_iffP P0 Ps : all_iff P0 Ps -> forall m n, nth P0 (P0 :: Ps) m <-> nth P0 (P0 :: Ps) n.