-- Mathlib/Geometry/Manifold/LocalInvariantProperties.lean
theorem liftPropWithinAt_mono_of_mem_nhdsWithin
    (mono_of_mem_nhdsWithin : ∀ ⦃s x t⦄ ⦃f : H → H'⦄, s ∈ 𝓝[t] x → P f s x → P f t x)
    (h : LiftPropWithinAt P g s x) (hst : s ∈ 𝓝[t] x) : LiftPropWithinAt P g t x

Proof state:
H : Type u_1
M : Type u_2
H' : Type u_3
M' : Type u_4
inst✝⁵ : TopologicalSpace H
inst✝⁴ : TopologicalSpace M
inst✝³ : ChartedSpace H M
inst✝² : TopologicalSpace H'
inst✝¹ : TopologicalSpace M'
inst✝ : ChartedSpace H' M'
P : (H → H') → Set H → H → Prop
g : M → M'
s t : Set M
x : M
mono_of_mem_nhdsWithin : ∀ ⦃s : Set H⦄ ⦃x : H⦄ ⦃t : Set H⦄ ⦃f : H → H'⦄, s ∈ 𝓝[t] x → P f s x → P f t x
hst : s ∈ 𝓝[t] x
h :
  ContinuousWithinAt g s x ∧
    P (↑(chartAt H' (g x)) ∘ g ∘ ↑(chartAt H x).symm) (↑(chartAt H x).symm ⁻¹' s) (↑(chartAt H x) x)
⊢ ↑(chartAt H x).symm ⁻¹' s ∈ 𝓝[↑(chartAt H x).symm ⁻¹' t] ↑(chartAt H x) x

Tactic:
simp_rw [← mem_map, (chartAt H x).symm.map_nhdsWithin_preimage_eq (mem_chart_target H x),
  (chartAt H x).left_inv (mem_chart_source H x), hst]

Result:
no goals

Proof state:
H : Type u_1
M : Type u_2
H' : Type u_3
M' : Type u_4
inst✝⁵ : TopologicalSpace H
inst✝⁴ : TopologicalSpace M
inst✝³ : ChartedSpace H M
inst✝² : TopologicalSpace H'
inst✝¹ : TopologicalSpace M'
inst✝ : ChartedSpace H' M'
P : (H → H') → Set H → H → Prop
g : M → M'
s t : Set M
x : M
mono_of_mem_nhdsWithin : ∀ ⦃s : Set H⦄ ⦃x : H⦄ ⦃t : Set H⦄ ⦃f : H → H'⦄, s ∈ 𝓝[t] x → P f s x → P f t x
h : LiftPropWithinAt P g s x
hst : s ∈ 𝓝[t] x
⊢ LiftPropWithinAt P g t x

Tactic:
simp only [liftPropWithinAt_iff'] at h ⊢

Result:
H : Type u_1
M : Type u_2
H' : Type u_3
M' : Type u_4
inst✝⁵ : TopologicalSpace H
inst✝⁴ : TopologicalSpace M
inst✝³ : ChartedSpace H M
inst✝² : TopologicalSpace H'
inst✝¹ : TopologicalSpace M'
inst✝ : ChartedSpace H' M'
P : (H → H') → Set H → H → Prop
g : M → M'
s t : Set M
x : M
mono_of_mem_nhdsWithin : ∀ ⦃s : Set H⦄ ⦃x : H⦄ ⦃t : Set H⦄ ⦃f : H → H'⦄, s ∈ 𝓝[t] x → P f s x → P f t x
hst : s ∈ 𝓝[t] x
h :
  ContinuousWithinAt g s x ∧
    P (↑(chartAt H' (g x)) ∘ g ∘ ↑(chartAt H x).symm) (↑(chartAt H x).symm ⁻¹' s) (↑(chartAt H x) x)
⊢ ContinuousWithinAt g t x ∧
    P (↑(chartAt H' (g x)) ∘ g ∘ ↑(chartAt H x).symm) (↑(chartAt H x).symm ⁻¹' t) (↑(chartAt H x) x)

Proof state:
H : Type u_1
M : Type u_2
H' : Type u_3
M' : Type u_4
inst✝⁵ : TopologicalSpace H
inst✝⁴ : TopologicalSpace M
inst✝³ : ChartedSpace H M
inst✝² : TopologicalSpace H'
inst✝¹ : TopologicalSpace M'
inst✝ : ChartedSpace H' M'
P : (H → H') → Set H → H → Prop
g : M → M'
s t : Set M
x : M
mono_of_mem_nhdsWithin : ∀ ⦃s : Set H⦄ ⦃x : H⦄ ⦃t : Set H⦄ ⦃f : H → H'⦄, s ∈ 𝓝[t] x → P f s x → P f t x
hst : s ∈ 𝓝[t] x
h :
  ContinuousWithinAt g s x ∧
    P (↑(chartAt H' (g x)) ∘ g ∘ ↑(chartAt H x).symm) (↑(chartAt H x).symm ⁻¹' s) (↑(chartAt H x) x)
⊢ ContinuousWithinAt g t x ∧
    P (↑(chartAt H' (g x)) ∘ g ∘ ↑(chartAt H x).symm) (↑(chartAt H x).symm ⁻¹' t) (↑(chartAt H x) x)

Tactic:
refine ⟨h.1.mono_of_mem_nhdsWithin hst, mono_of_mem_nhdsWithin ?_ h.2⟩

Result:
H : Type u_1
M : Type u_2
H' : Type u_3
M' : Type u_4
inst✝⁵ : TopologicalSpace H
inst✝⁴ : TopologicalSpace M
inst✝³ : ChartedSpace H M
inst✝² : TopologicalSpace H'
inst✝¹ : TopologicalSpace M'
inst✝ : ChartedSpace H' M'
P : (H → H') → Set H → H → Prop
g : M → M'
s t : Set M
x : M
mono_of_mem_nhdsWithin : ∀ ⦃s : Set H⦄ ⦃x : H⦄ ⦃t : Set H⦄ ⦃f : H → H'⦄, s ∈ 𝓝[t] x → P f s x → P f t x
hst : s ∈ 𝓝[t] x
h :
  ContinuousWithinAt g s x ∧
    P (↑(chartAt H' (g x)) ∘ g ∘ ↑(chartAt H x).symm) (↑(chartAt H x).symm ⁻¹' s) (↑(chartAt H x) x)
⊢ ↑(chartAt H x).symm ⁻¹' s ∈ 𝓝[↑(chartAt H x).symm ⁻¹' t] ↑(chartAt H x) x