Set: pp::colors
  Set: pp::unicode
  Imported 'tactic'
  Using: Nat
  Proved: add_assoc
Nat::add_zerol : ∀ a : ℕ, 0 + a = a
Nat::add_succl : ∀ a b : ℕ, a + 1 + b = a + b + 1
@eq_id : ∀ (A : (Type U)) (a : A), a = a ↔ ⊤
theorem add_assoc (a b c : ℕ) : a + (b + c) = a + b + c :=
    Nat::induction_on
        a
        (eqt_elim (trans (congr (congr2 eq (Nat::add_zerol (b + c))) (congr1 c (congr2 Nat::add (Nat::add_zerol b))))
                         (eq_id (b + c))))
        (λ (n : ℕ) (iH : n + (b + c) = n + b + c),
           eqt_elim (trans (congr (congr2 eq (trans (Nat::add_succl n (b + c)) (congr1 1 (congr2 Nat::add iH))))
                                  (trans (congr1 c (congr2 Nat::add (Nat::add_succl n b))) (Nat::add_succl (n + b) c)))
                           (eq_id (n + b + c + 1))))