simp only [Holds]
Δ : List Formula
F phi psi : Formula
D : Type
I : Interpretation D
V : VarAssignment D
E : Env
a✝ : ∀ H ∈ [], Holds D I V E H
⊢ Holds D I V E
    (((phi.iff_ psi).imp_ ((phi.imp_ psi).imp_ (psi.imp_ phi).not_).not_).imp_
        (((phi.imp_ psi).imp_ (psi.imp_ phi).not_).not_.imp_ (phi.iff_ psi)).not_).not_
Δ : List Formula
F phi psi : Formula
D : Type
I : Interpretation D
V : VarAssignment D
E : Env
a✝ : ∀ H ∈ [], Holds D I V E H
⊢ ¬(((Holds D I V E phi ↔ Holds D I V E psi) →
        ¬((Holds D I V E phi → Holds D I V E psi) → ¬(Holds D I V E psi → Holds D I V E phi))) →
      ¬(¬((Holds D I V E phi → Holds D I V E psi) → ¬(Holds D I V E psi → Holds D I V E phi)) →
          (Holds D I V E phi ↔ Holds D I V E psi)))