-- Mathlib/Geometry/Manifold/BumpFunction.lean
theorem nhdsWithin_range_basis :
    (𝓝[range I] extChartAt I c c).HasBasis (fun _ : SmoothBumpFunction I c => True) fun f =>
      closedBall (extChartAt I c c) f.rOut ∩ range I

Proof state:
E : Type uE
inst✝⁴ : NormedAddCommGroup E
inst✝³ : NormedSpace ℝ E
H : Type uH
inst✝² : TopologicalSpace H
I : ModelWithCorners ℝ E H
M : Type uM
inst✝¹ : TopologicalSpace M
inst✝ : ChartedSpace H M
c : M
⊢ (𝓝[range ↑I] ↑(extChartAt I c) c).HasBasis (fun x => True) fun f => closedBall (↑(extChartAt I c) c) f.rOut ∩ range ↑I

Tactic:
refine ((nhdsWithin_hasBasis nhds_basis_closedBall _).restrict_subset
  (extChartAt_target_mem_nhdsWithin _)).to_hasBasis' ?_ ?_

Result:
case refine_1
E : Type uE
inst✝⁴ : NormedAddCommGroup E
inst✝³ : NormedSpace ℝ E
H : Type uH
inst✝² : TopologicalSpace H
I : ModelWithCorners ℝ E H
M : Type uM
inst✝¹ : TopologicalSpace M
inst✝ : ChartedSpace H M
c : M
⊢ ∀ (i : ℝ),
    0 < i ∧ closedBall (↑(extChartAt I c) c) i ∩ range ↑I ⊆ (extChartAt I c).target →
      ∃ i', True ∧ closedBall (↑(extChartAt I c) c) i'.rOut ∩ range ↑I ⊆ closedBall (↑(extChartAt I c) c) i ∩ range ↑I

case refine_2
E : Type uE
inst✝⁴ : NormedAddCommGroup E
inst✝³ : NormedSpace ℝ E
H : Type uH
inst✝² : TopologicalSpace H
I : ModelWithCorners ℝ E H
M : Type uM
inst✝¹ : TopologicalSpace M
inst✝ : ChartedSpace H M
c : M
⊢ ∀ (i' : SmoothBumpFunction I c),
    True → closedBall (↑(extChartAt I c) c) i'.rOut ∩ range ↑I ∈ 𝓝[range ↑I] ↑(extChartAt I c) c

Proof state:
case refine_1.intro
E : Type uE
inst✝⁴ : NormedAddCommGroup E
inst✝³ : NormedSpace ℝ E
H : Type uH
inst✝² : TopologicalSpace H
I : ModelWithCorners ℝ E H
M : Type uM
inst✝¹ : TopologicalSpace M
inst✝ : ChartedSpace H M
c : M
R : ℝ
hR0 : 0 < R
hsub : closedBall (↑(extChartAt I c) c) R ∩ range ↑I ⊆ (extChartAt I c).target
⊢ ∃ i', True ∧ closedBall (↑(extChartAt I c) c) i'.rOut ∩ range ↑I ⊆ closedBall (↑(extChartAt I c) c) R ∩ range ↑I

Tactic:
exact ⟨⟨⟨R / 2, R, half_pos hR0, half_lt_self hR0⟩, hsub⟩, trivial, Subset.rfl⟩

Result:
no goals

Proof state:
case refine_2
E : Type uE
inst✝⁴ : NormedAddCommGroup E
inst✝³ : NormedSpace ℝ E
H : Type uH
inst✝² : TopologicalSpace H
I : ModelWithCorners ℝ E H
M : Type uM
inst✝¹ : TopologicalSpace M
inst✝ : ChartedSpace H M
c : M
⊢ ∀ (i' : SmoothBumpFunction I c),
    True → closedBall (↑(extChartAt I c) c) i'.rOut ∩ range ↑I ∈ 𝓝[range ↑I] ↑(extChartAt I c) c

Tactic:
exact fun f _ => inter_mem (mem_nhdsWithin_of_mem_nhds <| closedBall_mem_nhds _ f.rOut_pos)
  self_mem_nhdsWithin

Result:
no goals

Proof state:
case refine_1
E : Type uE
inst✝⁴ : NormedAddCommGroup E
inst✝³ : NormedSpace ℝ E
H : Type uH
inst✝² : TopologicalSpace H
I : ModelWithCorners ℝ E H
M : Type uM
inst✝¹ : TopologicalSpace M
inst✝ : ChartedSpace H M
c : M
⊢ ∀ (i : ℝ),
    0 < i ∧ closedBall (↑(extChartAt I c) c) i ∩ range ↑I ⊆ (extChartAt I c).target →
      ∃ i', True ∧ closedBall (↑(extChartAt I c) c) i'.rOut ∩ range ↑I ⊆ closedBall (↑(extChartAt I c) c) i ∩ range ↑I

Tactic:
rintro R ⟨hR0, hsub⟩

Result:
case refine_1.intro
E : Type uE
inst✝⁴ : NormedAddCommGroup E
inst✝³ : NormedSpace ℝ E
H : Type uH
inst✝² : TopologicalSpace H
I : ModelWithCorners ℝ E H
M : Type uM
inst✝¹ : TopologicalSpace M
inst✝ : ChartedSpace H M
c : M
R : ℝ
hR0 : 0 < R
hsub : closedBall (↑(extChartAt I c) c) R ∩ range ↑I ⊆ (extChartAt I c).target
⊢ ∃ i', True ∧ closedBall (↑(extChartAt I c) c) i'.rOut ∩ range ↑I ⊆ closedBall (↑(extChartAt I c) c) R ∩ range ↑I