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]
    · constructor
      · simp
      · decide
    · exact ⟨rfl, inferInstance⟩
    · have := opow_pos (repr a') omega0_pos
      refine
        ⟨isSuccLimit_mul_right this isSuccLimit_omega0, fun i =>
          ⟨this, ?_, fun H => @NF.oadd_zero _ _ (iha.2 H.fst)⟩, exists_lt_mul_omega0'⟩
      rw [← mul_add_one, ← Nat.cast_add_one]
      gcongr
      apply natCast_lt_omega0
    · have := opow_pos (repr a') omega0_pos
      refine
        ⟨isSuccLimit_add _ (isSuccLimit_mul_right this isSuccLimit_omega0), fun i => ⟨this, ?_, ?_⟩,
          exists_lt_add exists_lt_mul_omega0'⟩
      · rw [← mul_add_one, ← Nat.cast_add_one]
        gcongr
        apply natCast_lt_omega0
      · refine fun H => H.fst.oadd _ (NF.below_of_lt' ?_ (@NF.oadd_zero _ _ (iha.2 H.fst)))
        rw [repr, repr_zero, add_zero, iha.1, opow_succ]
        gcongr
        apply natCast_lt_omega0
    · rcases iha with ⟨h1, h2, h3⟩
      refine ⟨isSuccLimit_opow one_lt_omega0 h1, fun i => ?_,
        exists_lt_omega0_opow' one_lt_omega0 h1 h3⟩
      obtain ⟨h4, h5, h6⟩ := h2 i
      exact ⟨h4, h5, fun H => @NF.oadd_zero _ _ (h6 H.fst)⟩
    · rcases iha with ⟨h1, h2, h3⟩
      refine
        ⟨isSuccLimit_add _ (isSuccLimit_opow one_lt_omega0 h1), fun i => ?_,
          exists_lt_add (exists_lt_omega0_opow' one_lt_omega0 h1 h3)⟩
      obtain ⟨h4, h5, h6⟩ := h2 i
      refine ⟨h4, h5, fun H => H.fst.oadd _ (NF.below_of_lt' ?_ (@NF.oadd_zero _ _ (h6 H.fst)))⟩
      rwa [repr, repr_zero, add_zero, PNat.one_coe, Nat.cast_one, mul_one,
        opow_lt_opow_iff_right one_lt_omega0]

Goal state after: