/-- The units of the stalk at `x` of the sheaf of smooth functions from `M` to `𝕜`, considered as a
sheaf of commutative rings, are the functions whose values at `x` are nonzero. -/
theorem smoothSheafCommRing.isUnit_stalk_iff {x : M}
    (f : (smoothSheafCommRing IM 𝓘(𝕜) M 𝕜).presheaf.stalk x) :
    IsUnit f ↔ f ∉ RingHom.ker (smoothSheafCommRing.eval IM 𝓘(𝕜) M 𝕜 x) := by
  constructor
  · rintro ⟨⟨f, g, hf, hg⟩, rfl⟩ (h' : smoothSheafCommRing.eval IM 𝓘(𝕜) M 𝕜 x f = 0)
    simpa [h'] using congr_arg (smoothSheafCommRing.eval IM 𝓘(𝕜) M 𝕜 x) hf
  · let S := (smoothSheafCommRing IM 𝓘(𝕜) M 𝕜).presheaf
    -- Suppose that `f`, in the stalk at `x`, is nonzero at `x`
    rintro (hf : _ ≠ 0)
    -- Represent `f` as the germ of some function (also called `f`) on an open neighbourhood `U` of
    -- `x`, which is nonzero at `x`
    obtain ⟨U : Opens M, hxU, f : C^∞⟮IM, U; 𝓘(𝕜), 𝕜⟯, rfl⟩ := S.germ_exist x f
    have hf' : f ⟨x, hxU⟩ ≠ 0 := by
      convert hf
      exact (smoothSheafCommRing.eval_germ U ⟨x, hxU⟩ f).symm
    -- In fact, by continuity, `f` is nonzero on a neighbourhood `V` of `x`
    have H :  ∀ᶠ (z : U) in 𝓝 ⟨x, hxU⟩, f z ≠ 0 := f.2.continuous.continuousAt.eventually_ne hf'
    rw [eventually_nhds_iff] at H
    obtain ⟨V₀, hV₀f, hV₀, hxV₀⟩ := H
    let V : Opens M := ⟨Subtype.val '' V₀, U.2.isOpenMap_subtype_val V₀ hV₀⟩
    have hUV : V ≤ U := Subtype.coe_image_subset (U : Set M) V₀
    have hV : V₀ = Set.range (Set.inclusion hUV) := by
      convert (Set.range_inclusion hUV).symm
      ext y
      show _ ↔ y ∈ Subtype.val ⁻¹' (Subtype.val '' V₀)
      rw [Set.preimage_image_eq _ Subtype.coe_injective]
    clear_value V
    subst hV
    have hxV : x ∈ (V : Set M) := by
      obtain ⟨x₀, hxx₀⟩ := hxV₀
      convert x₀.2
      exact congr_arg Subtype.val hxx₀.symm
    have hVf : ∀ y : V, f (Set.inclusion hUV y) ≠ 0 :=
      fun y ↦ hV₀f (Set.inclusion hUV y) (Set.mem_range_self y)
    -- Let `g` be the pointwise inverse of `f` on `V`, which is smooth since `f` is nonzero there
    let g : C^∞⟮IM, V; 𝓘(𝕜), 𝕜⟯ := ⟨(f ∘ Set.inclusion hUV)⁻¹, ?_⟩
    -- The germ of `g` is inverse to the germ of `f`, so `f` is a unit
    · refine ⟨⟨S.germ ⟨x, hxV⟩ (SmoothMap.restrictRingHom IM 𝓘(𝕜) 𝕜 hUV f), S.germ ⟨x, hxV⟩ g,
        ?_, ?_⟩, S.germ_res_apply hUV.hom ⟨x, hxV⟩ f⟩
      · rw [← map_mul]
        convert map_one _
        apply Subtype.ext
        ext y
        apply mul_inv_cancel
        exact hVf y
      · rw [← map_mul]
        convert map_one _
        apply Subtype.ext
        ext y
        apply inv_mul_cancel
        exact hVf y
    · 

Proof state:
case mpr.intro.intro.intro.intro.intro.intro.refine_1
𝕜 : Type u
inst✝⁶ : NontriviallyNormedField 𝕜
inst✝⁵ : CompleteSpace 𝕜
EM : Type u_1
inst✝⁴ : NormedAddCommGroup EM
inst✝³ : NormedSpace 𝕜 EM
HM : Type u_2
inst✝² : TopologicalSpace HM
IM : ModelWithCorners 𝕜 EM HM
M : Type u
inst✝¹ : TopologicalSpace M
inst✝ : ChartedSpace HM M
x : M
S : TopCat.Presheaf CommRingCat (TopCat.of M) := TopCat.Sheaf.presheaf (smoothSheafCommRing IM 𝓘(𝕜, 𝕜) M 𝕜)
U : Opens M
hxU : x ∈ U
f : C^⊤⟮IM, ↥U; 𝓘(𝕜, 𝕜), 𝕜⟯
hf : (eval IM 𝓘(𝕜, 𝕜) M 𝕜 x) ((TopCat.Presheaf.germ S { val := x, property := hxU }) f) ≠ 0
hf' : f { val := x, property := hxU } ≠ 0
V : Opens M
hUV : V ≤ U
hV₀f : ∀ x ∈ Set.range (Set.inclusion hUV), f x ≠ 0
hV₀ : IsOpen (Set.range (Set.inclusion hUV))
hxV₀ : { val := x, property := hxU } ∈ Set.range (Set.inclusion hUV)
hxV : x ∈ ↑V
hVf : ∀ (y : ↥V), f (Set.inclusion hUV y) ≠ 0
⊢ ContMDiff IM 𝓘(𝕜, 𝕜) ⊤ (⇑f ∘ Set.inclusion hUV)⁻¹

Next tactic:
intro y