-- Mathlib/Analysis/Meromorphic/Order.lean
/-- The order of a meromorphic function `f` at `z₀` equals an integer `n` iff `f` can locally be
written as `f z = (z - z₀) ^ n • g z`, where `g` is analytic and does not vanish at `z₀`. -/
lemma order_eq_int_iff {f : 𝕜 → E} {x : 𝕜} (hf : MeromorphicAt f x) (n : ℤ) : hf.order = n ↔
    ∃ g : 𝕜 → E, AnalyticAt 𝕜 g x ∧ g x ≠ 0 ∧ ∀ᶠ z in 𝓝[≠] x, f z = (z - x) ^ n • g z

Proof state:
case pos
𝕜 : Type u_1
inst✝² : NontriviallyNormedField 𝕜
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace 𝕜 E
f : 𝕜 → E
x : 𝕜
hf : MeromorphicAt f x
n : ℤ
h : ∀ᶠ (z : 𝕜) in 𝓝 x, (z - x) ^ Exists.choose hf • f z = 0
⊢ ¬∃ g, AnalyticAt 𝕜 g x ∧ g x ≠ 0 ∧ ∀ᶠ (z : 𝕜) in 𝓝[≠] x, f z = (z - x) ^ n • g z

Tactic:
refine fun ⟨g, hg_an, hg_ne, hg_eq⟩ ↦ hg_ne ?_

Result:
case pos
𝕜 : Type u_1
inst✝² : NontriviallyNormedField 𝕜
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace 𝕜 E
f : 𝕜 → E
x : 𝕜
hf : MeromorphicAt f x
n : ℤ
h : ∀ᶠ (z : 𝕜) in 𝓝 x, (z - x) ^ Exists.choose hf • f z = 0
x✝ : ∃ g, AnalyticAt 𝕜 g x ∧ g x ≠ 0 ∧ ∀ᶠ (z : 𝕜) in 𝓝[≠] x, f z = (z - x) ^ n • g z
g : 𝕜 → E
hg_an : AnalyticAt 𝕜 g x
hg_ne : g x ≠ 0
hg_eq : ∀ᶠ (z : 𝕜) in 𝓝[≠] x, f z = (z - x) ^ n • g z
⊢ g x = 0

Proof state:
case neg.intro
𝕜 : Type u_1
inst✝² : NontriviallyNormedField 𝕜
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace 𝕜 E
f : 𝕜 → E
x : 𝕜
hf : MeromorphicAt f x
n : ℤ
h✝ : ¬⋯.order = ⊤
m : ℕ
h : ↑m = ⋯.order
⊢ ↑m - ↑(Exists.choose hf) = n ↔ ∃ g, AnalyticAt 𝕜 g x ∧ g x ≠ 0 ∧ ∀ᶠ (z : 𝕜) in 𝓝[≠] x, f z = (z - x) ^ n • g z

Tactic:
obtain ⟨g, hg_an, hg_ne, hg_eq⟩ := (AnalyticAt.order_eq_nat_iff _ _).mp h.symm

Result:
case neg.intro.intro.intro.intro
𝕜 : Type u_1
inst✝² : NontriviallyNormedField 𝕜
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace 𝕜 E
f : 𝕜 → E
x : 𝕜
hf : MeromorphicAt f x
n : ℤ
h✝ : ¬⋯.order = ⊤
m : ℕ
h : ↑m = ⋯.order
g : 𝕜 → E
hg_an : AnalyticAt 𝕜 g x
hg_ne : g x ≠ 0
hg_eq : ∀ᶠ (z : 𝕜) in 𝓝 x, (z - x) ^ Exists.choose hf • f z = (z - x) ^ m • g z
⊢ ↑m - ↑(Exists.choose hf) = n ↔ ∃ g, AnalyticAt 𝕜 g x ∧ g x ≠ 0 ∧ ∀ᶠ (z : 𝕜) in 𝓝[≠] x, f z = (z - x) ^ n • g z

Proof state:
𝕜 : Type u_1
inst✝² : NontriviallyNormedField 𝕜
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace 𝕜 E
f : 𝕜 → E
x : 𝕜
hf : MeromorphicAt f x
n : ℤ
⊢ hf.order = ↑n ↔ ∃ g, AnalyticAt 𝕜 g x ∧ g x ≠ 0 ∧ ∀ᶠ (z : 𝕜) in 𝓝[≠] x, f z = (z - x) ^ n • g z

Tactic:
unfold order

Result:
𝕜 : Type u_1
inst✝² : NontriviallyNormedField 𝕜
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace 𝕜 E
f : 𝕜 → E
x : 𝕜
hf : MeromorphicAt f x
n : ℤ
⊢ ENat.map (fun x => ↑x) ⋯.order - ↑(Exists.choose hf) = ↑n ↔
    ∃ g, AnalyticAt 𝕜 g x ∧ g x ≠ 0 ∧ ∀ᶠ (z : 𝕜) in 𝓝[≠] x, f z = (z - x) ^ n • g z

Proof state:
case pos
𝕜 : Type u_1
inst✝² : NontriviallyNormedField 𝕜
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace 𝕜 E
f : 𝕜 → E
x : 𝕜
hf : MeromorphicAt f x
n : ℤ
h : ⋯.order = ⊤
⊢ ENat.map (fun x => ↑x) ⋯.order - ↑(Exists.choose hf) = ↑n ↔
    ∃ g, AnalyticAt 𝕜 g x ∧ g x ≠ 0 ∧ ∀ᶠ (z : 𝕜) in 𝓝[≠] x, f z = (z - x) ^ n • g z

Tactic:
rw [h, ENat.map_top, ← WithTop.coe_natCast, top_sub,
  eq_false_intro WithTop.top_ne_coe, false_iff]

Result:
case pos
𝕜 : Type u_1
inst✝² : NontriviallyNormedField 𝕜
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace 𝕜 E
f : 𝕜 → E
x : 𝕜
hf : MeromorphicAt f x
n : ℤ
h : ⋯.order = ⊤
⊢ ¬∃ g, AnalyticAt 𝕜 g x ∧ g x ≠ 0 ∧ ∀ᶠ (z : 𝕜) in 𝓝[≠] x, f z = (z - x) ^ n • g z