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]])

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
⊢ (a.oadd 1 b).repr = succ zero.repr ∧ ((a.oadd 1 b).NF → zero.NF)

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
⊢ (a.oadd (m' + 1).succPNat b).repr = succ (zero.oadd m'.succPNat zero).repr ∧
    ((a.oadd (m' + 1).succPNat b).NF → (zero.oadd m'.succPNat zero).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 (a.oadd 1 b).repr ∧
    (∀ (i : ℕ),
        a'.oadd i.succPNat zero < a'.oadd (i + 1).succPNat zero ∧
          a'.oadd i.succPNat zero < a.oadd 1 b ∧ ((a.oadd 1 b).NF → (a'.oadd i.succPNat zero).NF)) ∧
      ∀ a_1 < (a.oadd 1 b).repr, ∃ i, a_1 < (a'.oadd i.succPNat zero).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 (a.oadd (m' + 1).succPNat b).repr ∧
    (∀ (i : ℕ),
        a.oadd m'.succPNat (a'.oadd i.succPNat zero) < a.oadd m'.succPNat (a'.oadd (i + 1).succPNat zero) ∧
          a.oadd m'.succPNat (a'.oadd i.succPNat zero) < a.oadd (m' + 1).succPNat b ∧
            ((a.oadd (m' + 1).succPNat b).NF → (a.oadd m'.succPNat (a'.oadd i.succPNat zero)).NF)) ∧
      ∀ a_1 < (a.oadd (m' + 1).succPNat b).repr, ∃ i, a_1 < (a.oadd m'.succPNat (a'.oadd i.succPNat zero)).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 (a.oadd 1 b).repr ∧
    (∀ (i : ℕ),
        (f i).oadd 1 zero < (f (i + 1)).oadd 1 zero ∧
          (f i).oadd 1 zero < a.oadd 1 b ∧ ((a.oadd 1 b).NF → ((f i).oadd 1 zero).NF)) ∧
      ∀ a_1 < (a.oadd 1 b).repr, ∃ i, a_1 < ((f i).oadd 1 zero).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 (a.oadd (m' + 1).succPNat b).repr ∧
    (∀ (i : ℕ),
        a.oadd m'.succPNat ((f i).oadd 1 zero) < a.oadd m'.succPNat ((f (i + 1)).oadd 1 zero) ∧
          a.oadd m'.succPNat ((f i).oadd 1 zero) < a.oadd (m' + 1).succPNat b ∧
            ((a.oadd (m' + 1).succPNat b).NF → (a.oadd m'.succPNat ((f i).oadd 1 zero)).NF)) ∧
      ∀ a_1 < (a.oadd (m' + 1).succPNat b).repr, ∃ i, a_1 < (a.oadd m'.succPNat ((f i).oadd 1 zero)).repr