-- Mathlib/NumberTheory/Padics/PadicNumbers.lean
theorem rat_dense' (q : ℚ_[p]) {ε : ℚ} (hε : 0 < ε) : ∃ r : ℚ, padicNormE (q - r : ℚ_[p]) < ε :=
  Quotient.inductionOn q fun q' ↦
    have : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (q' m - q' n) < ε := cauchy₂ _ hε
    let ⟨N, hN⟩ := this
    ⟨q' N, by
      classical
      dsimp [padicNormE]
      -- Porting note: `change` → `convert_to` (`change` times out!)
      -- and add `PadicSeq p` type annotation
      convert_to PadicSeq.norm (q' - const _ (q' N) : PadicSeq p) < ε
      rcases Decidable.em (q' - const (padicNorm p) (q' N) ≈ 0) with heq | hne'
      · simpa only [heq, PadicSeq.norm, dif_pos]
      · simp only [PadicSeq.norm, dif_neg hne']
        change padicNorm p (q' _ - q' _) < ε
        rcases Decidable.em (stationaryPoint hne' ≤ N) with hle | hle
        · -- Porting note: inlined `stationaryPoint_spec` invocation.
          have := (stationaryPoint_spec hne' le_rfl hle).symm
          simp only [const_apply, sub_apply, padicNorm.zero, sub_self] at this
          simpa only [this]
        · exact hN _ (lt_of_not_ge hle).le _ le_rfl⟩

Proof state:
case inr
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
hne' : ¬q' - const (padicNorm p) (↑q' N) ≈ 0
⊢ (q' - const (padicNorm p) (↑q' N)).norm < ε

Tactic:
simp only [PadicSeq.norm, dif_neg hne']

Result:
case inr
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
hne' : ¬q' - const (padicNorm p) (↑q' N) ≈ 0
⊢ padicNorm p (↑(q' - const (padicNorm p) (↑q' N)) (stationaryPoint hne')) < ε

Proof state:
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
⊢ (q' - const (padicNorm p) (↑q' N)).norm < ε

Tactic:
rcases Decidable.em (q' - const (padicNorm p) (q' N) ≈ 0) with heq | hne'

Result:
case inl
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
heq : q' - const (padicNorm p) (↑q' N) ≈ 0
⊢ (q' - const (padicNorm p) (↑q' N)).norm < ε

case inr
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
hne' : ¬q' - const (padicNorm p) (↑q' N) ≈ 0
⊢ (q' - const (padicNorm p) (↑q' N)).norm < ε

Proof state:
case inr.inl
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this✝ : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
hne' : ¬q' - const (padicNorm p) (↑q' N) ≈ 0
hle : stationaryPoint hne' ≤ N
this : padicNorm p (↑q' (stationaryPoint hne') - ↑q' N) = 0
⊢ padicNorm p (↑q' (stationaryPoint hne') - ↑q' N) < ε

Tactic:
simpa only [this]

Result:
no goals

Proof state:
case inr.inl
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
hne' : ¬q' - const (padicNorm p) (↑q' N) ≈ 0
hle : stationaryPoint hne' ≤ N
⊢ padicNorm p (↑q' (stationaryPoint hne') - ↑q' N) < ε

Tactic:
have := (stationaryPoint_spec hne' le_rfl hle).symm

Result:
case inr.inl
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this✝ : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
hne' : ¬q' - const (padicNorm p) (↑q' N) ≈ 0
hle : stationaryPoint hne' ≤ N
this :
  padicNorm p (↑(q' - const (padicNorm p) (↑q' N)) (stationaryPoint hne')) =
    padicNorm p (↑(q' - const (padicNorm p) (↑q' N)) N)
⊢ padicNorm p (↑q' (stationaryPoint hne') - ↑q' N) < ε

Proof state:
case inr
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
hne' : ¬q' - const (padicNorm p) (↑q' N) ≈ 0
⊢ padicNorm p (↑q' (stationaryPoint hne') - ↑q' N) < ε

Tactic:
rcases Decidable.em (stationaryPoint hne' ≤ N) with hle | hle

Result:
case inr.inl
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
hne' : ¬q' - const (padicNorm p) (↑q' N) ≈ 0
hle : stationaryPoint hne' ≤ N
⊢ padicNorm p (↑q' (stationaryPoint hne') - ↑q' N) < ε

case inr.inr
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
hne' : ¬q' - const (padicNorm p) (↑q' N) ≈ 0
hle : ¬stationaryPoint hne' ≤ N
⊢ padicNorm p (↑q' (stationaryPoint hne') - ↑q' N) < ε

Proof state:
case inl
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
heq : q' - const (padicNorm p) (↑q' N) ≈ 0
⊢ (q' - const (padicNorm p) (↑q' N)).norm < ε

Tactic:
simpa only [heq, PadicSeq.norm, dif_pos]

Result:
no goals

Proof state:
case inr.inr
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
hne' : ¬q' - const (padicNorm p) (↑q' N) ≈ 0
hle : ¬stationaryPoint hne' ≤ N
⊢ padicNorm p (↑q' (stationaryPoint hne') - ↑q' N) < ε

Tactic:
exact hN _ (lt_of_not_ge hle).le _ le_rfl

Result:
no goals

Proof state:
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
⊢ Quotient.lift PadicSeq.norm ⋯ (Completion.mk q' - ↑(↑q' N)) < ε

Tactic:
convert_to PadicSeq.norm (q' - const _ (q' N) : PadicSeq p) < ε

Result:
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
⊢ (q' - const (padicNorm p) (↑q' N)).norm < ε

Proof state:
case inr
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
hne' : ¬q' - const (padicNorm p) (↑q' N) ≈ 0
⊢ padicNorm p (↑(q' - const (padicNorm p) (↑q' N)) (stationaryPoint hne')) < ε

Tactic:
change padicNorm p (q' _ - q' _) < ε

Result:
case inr
p : ℕ
inst✝ : Fact (Nat.Prime p)
q : ℚ_[p]
ε : ℚ
hε : 0 < ε
q' : CauSeq ℚ (padicNorm p)
this : ∃ N, ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
N : ℕ
hN : ∀ m ≥ N, ∀ n ≥ N, padicNorm p (↑q' m - ↑q' n) < ε
hne' : ¬q' - const (padicNorm p) (↑q' N) ≈ 0
⊢ padicNorm p (↑q' (stationaryPoint hne') - ↑q' N) < ε