Module: Mathlib.SetTheory.Ordinal.Notation

Goal state before:
pp: case oadd.inl.none
a : ONote
m : ℕ+
b : ONote
iha : a.FundamentalSequenceProp a.fundamentalSequence
ihb : b = 0
e : b.fundamentalSequence = Sum.inl none
⊢ match
    match a.fundamentalSequence, m.natPred with
    | Sum.inl none, 0 => Sum.inl (some zero)
    | Sum.inl none, m.succ => Sum.inl (some (zero.oadd m.succPNat zero))
    | Sum.inl (some a'), 0 => Sum.inr fun i ↦ a'.oadd i.succPNat zero
    | Sum.inl (some a'), m.succ => Sum.inr fun i ↦ a.oadd m.succPNat (a'.oadd i.succPNat zero)
    | Sum.inr f, 0 => Sum.inr fun i ↦ (f i).oadd 1 zero
    | Sum.inr f, m.succ => Sum.inr fun i ↦ a.oadd m.succPNat ((f i).oadd 1 zero) with
  | Sum.inl none => a.oadd m b = 0
  | Sum.inl (some a_1) => (a.oadd m b).repr = succ a_1.repr ∧ ((a.oadd m b).NF → a_1.NF)
  | Sum.inr f =>
    IsSuccLimit (a.oadd m b).repr ∧
      (∀ (i : ℕ), f i < f (i + 1) ∧ f i < a.oadd m b ∧ ((a.oadd m b).NF → (f i).NF)) ∧
        ∀ a_1 < (a.oadd m b).repr, ∃ i, a_1 < (f i).repr

Tactic:
rcases e : a.fundamentalSequence with (⟨_ | a'⟩ | f) <;> rcases e' : m.natPred with - | m' <;>
      simp only <;>
      rw [e, FundamentalSequenceProp] at iha <;>
      (try rw [show m = 1 by
            have := PNat.natPred_add_one m; rw [e'] at this; exact PNat.coe_inj.1 this.symm]) <;>
      (try rw [show m = (m' + 1).succPNat by
              rw [← e', ← PNat.coe_inj, Nat.succPNat_coe, ← Nat.add_one, PNat.natPred_add_one]]) <;>
      simp only [repr, repr_zero, iha, ihb, opow_lt_opow_iff_right one_lt_omega0,
        add_lt_add_iff_left, add_zero, lt_add_iff_pos_right, lt_def, mul_one, Nat.cast_zero,
        Nat.cast_succ, Nat.succPNat_coe, opow_succ, opow_zero, mul_add_one, PNat.one_coe,
        _root_.zero_add, zero_def]

Goal state after:
case oadd.inl.none.inl.none.zero
a : ONote
m : ℕ+
b : ONote
iha : a = 0
ihb : b = 0
e✝ : b.fundamentalSequence = Sum.inl none
e : a.fundamentalSequence = Sum.inl none
e' : m.natPred = 0
⊢ 1 = succ 0 ∧ ((oadd 0 1 0).NF → NF 0)

case oadd.inl.none.inl.none.succ
a : ONote
m : ℕ+
b : ONote
iha : a = 0
ihb : b = 0
e✝ : b.fundamentalSequence = Sum.inl none
e : a.fundamentalSequence = Sum.inl none
m' : ℕ
e' : m.natPred = m' + 1
⊢ 1 * ↑m' + 1 + 1 = succ (1 * ↑m' + 1) ∧ ((oadd 0 (m' + 1).succPNat 0).NF → (oadd 0 m'.succPNat 0).NF)

case oadd.inl.none.inl.some.zero
a : ONote
m : ℕ+
b : ONote
ihb : b = 0
e✝ : b.fundamentalSequence = Sum.inl none
a' : ONote
iha : a.repr = succ a'.repr ∧ (a.NF → a'.NF)
e : a.fundamentalSequence = Sum.inl (some a')
e' : m.natPred = 0
⊢ IsSuccLimit (ω ^ a'.repr * ω) ∧
    (∀ (i : ℕ),
        0 < ω ^ a'.repr ∧
          ω ^ a'.repr * ↑i + ω ^ a'.repr < ω ^ a'.repr * ω ∧ ((a.oadd 1 0).NF → (a'.oadd i.succPNat 0).NF)) ∧
      ∀ a < ω ^ a'.repr * ω, ∃ i, a < ω ^ a'.repr * ↑i + ω ^ a'.repr

case oadd.inl.none.inl.some.succ
a : ONote
m : ℕ+
b : ONote
ihb : b = 0
e✝ : b.fundamentalSequence = Sum.inl none
a' : ONote
iha : a.repr = succ a'.repr ∧ (a.NF → a'.NF)
e : a.fundamentalSequence = Sum.inl (some a')
m' : ℕ
e' : m.natPred = m' + 1
⊢ IsSuccLimit (ω ^ a'.repr * ω * ↑m' + ω ^ a'.repr * ω + ω ^ a'.repr * ω) ∧
    (∀ (i : ℕ),
        0 < ω ^ a'.repr ∧
          ω ^ a'.repr * ↑i + ω ^ a'.repr < ω ^ a'.repr * ω ∧
            ((a.oadd (m' + 1).succPNat 0).NF → (a.oadd m'.succPNat (a'.oadd i.succPNat 0)).NF)) ∧
      ∀ a < ω ^ a'.repr * ω * ↑m' + ω ^ a'.repr * ω + ω ^ a'.repr * ω,
        ∃ i, a < ω ^ a'.repr * ω * ↑m' + ω ^ a'.repr * ω + (ω ^ a'.repr * ↑i + ω ^ a'.repr)

case oadd.inl.none.inr.zero
a : ONote
m : ℕ+
b : ONote
ihb : b = 0
e✝ : b.fundamentalSequence = Sum.inl none
f : ℕ → ONote
iha :
  IsSuccLimit a.repr ∧
    (∀ (i : ℕ), f i < f (i + 1) ∧ f i < a ∧ (a.NF → (f i).NF)) ∧ ∀ a_1 < a.repr, ∃ i, a_1 < (f i).repr
e : a.fundamentalSequence = Sum.inr f
e' : m.natPred = 0
⊢ IsSuccLimit (ω ^ a.repr) ∧
    (∀ (i : ℕ), (f i).repr < (f (i + 1)).repr ∧ (f i).repr < a.repr ∧ ((a.oadd 1 0).NF → ((f i).oadd 1 0).NF)) ∧
      ∀ a_1 < ω ^ a.repr, ∃ i, a_1 < ω ^ (f i).repr

case oadd.inl.none.inr.succ
a : ONote
m : ℕ+
b : ONote
ihb : b = 0
e✝ : b.fundamentalSequence = Sum.inl none
f : ℕ → ONote
iha :
  IsSuccLimit a.repr ∧
    (∀ (i : ℕ), f i < f (i + 1) ∧ f i < a ∧ (a.NF → (f i).NF)) ∧ ∀ a_1 < a.repr, ∃ i, a_1 < (f i).repr
e : a.fundamentalSequence = Sum.inr f
m' : ℕ
e' : m.natPred = m' + 1
⊢ IsSuccLimit (ω ^ a.repr * ↑m' + ω ^ a.repr + ω ^ a.repr) ∧
    (∀ (i : ℕ),
        (f i).repr < (f (i + 1)).repr ∧
          (f i).repr < a.repr ∧ ((a.oadd (m' + 1).succPNat 0).NF → (a.oadd m'.succPNat ((f i).oadd 1 0)).NF)) ∧
      ∀ a_1 < ω ^ a.repr * ↑m' + ω ^ a.repr + ω ^ a.repr, ∃ i, a_1 < ω ^ a.repr * ↑m' + ω ^ a.repr + ω ^ (f i).repr