refine Nat.strong_induction_on N ?_
W : Type
M : KripkeModel W
w : W
N : ℕ
⊢ ∀ (XS : List (List Formula)), XS.length = N → (Evaluate (M, w) (discon XS) ↔ ∃ Y ∈ XS, ∀ f ∈ Y, Evaluate (M, w) f)
W : Type
M : KripkeModel W
w : W
N : ℕ
⊢ ∀ (n : ℕ),
    (∀ m < n,
        ∀ (XS : List (List Formula)),
          XS.length = m → (Evaluate (M, w) (discon XS) ↔ ∃ Y ∈ XS, ∀ f ∈ Y, Evaluate (M, w) f)) →
      ∀ (XS : List (List Formula)), XS.length = n → (Evaluate (M, w) (discon XS) ↔ ∃ Y ∈ XS, ∀ f ∈ Y, Evaluate (M, w) f)