-- Mathlib/RingTheory/Spectrum/Prime/FreeLocus.lean
lemma isLocallyConstant_rankAtStalk_freeLocus [Module.FinitePresentation R M] :
    IsLocallyConstant (fun x : freeLocus R M ↦ rankAtStalk M x.1)

Proof state:
case intro.intro.intro.mk
R : Type uR
M : Type uM
inst✝³ : CommRing R
inst✝² : AddCommGroup M
inst✝¹ : Module R M
inst✝ : FinitePresentation R M
x✝ : ↑(freeLocus R M)
x : PrimeSpectrum R
hx : x ∈ freeLocus R M
this : Free (Localization.AtPrime x.asIdeal) (LocalizedModule x.asIdeal.primeCompl M)
f : R
hf : f ∈ x.asIdeal.primeCompl
hf' : Free (Localization (Submonoid.powers f)) (LocalizedModule (Submonoid.powers f) M)
hf'' :
  finrank (Localization (Submonoid.powers f)) (LocalizedModule (Submonoid.powers f) M) =
    finrank (Localization.AtPrime x.asIdeal) (LocalizedModule x.asIdeal.primeCompl M)
p : PrimeSpectrum R
hp'' : p ∈ freeLocus R M
hp : ⟨p, hp''⟩ ∈ Subtype.val ⁻¹' ↑(basicOpen f)
p' : Submonoid (Localization (Submonoid.powers f)) :=
  Algebra.algebraMapSubmonoid (Localization (Submonoid.powers f)) p.asIdeal.primeCompl
hp' : Submonoid.powers f ≤ p.asIdeal.primeCompl
Rₚ : Type uR := Localization.AtPrime p.asIdeal
⊢ rankAtStalk M ↑⟨p, hp''⟩ = rankAtStalk M ↑⟨x, hx⟩

Tactic:
let Mₚ := LocalizedModule p.asIdeal.primeCompl M

Result:
case intro.intro.intro.mk
R : Type uR
M : Type uM
inst✝³ : CommRing R
inst✝² : AddCommGroup M
inst✝¹ : Module R M
inst✝ : FinitePresentation R M
x✝ : ↑(freeLocus R M)
x : PrimeSpectrum R
hx : x ∈ freeLocus R M
this : Free (Localization.AtPrime x.asIdeal) (LocalizedModule x.asIdeal.primeCompl M)
f : R
hf : f ∈ x.asIdeal.primeCompl
hf' : Free (Localization (Submonoid.powers f)) (LocalizedModule (Submonoid.powers f) M)
hf'' :
  finrank (Localization (Submonoid.powers f)) (LocalizedModule (Submonoid.powers f) M) =
    finrank (Localization.AtPrime x.asIdeal) (LocalizedModule x.asIdeal.primeCompl M)
p : PrimeSpectrum R
hp'' : p ∈ freeLocus R M
hp : ⟨p, hp''⟩ ∈ Subtype.val ⁻¹' ↑(basicOpen f)
p' : Submonoid (Localization (Submonoid.powers f)) :=
  Algebra.algebraMapSubmonoid (Localization (Submonoid.powers f)) p.asIdeal.primeCompl
hp' : Submonoid.powers f ≤ p.asIdeal.primeCompl
Rₚ : Type uR := Localization.AtPrime p.asIdeal
Mₚ : Type (max uR uM) := LocalizedModule p.asIdeal.primeCompl M
⊢ rankAtStalk M ↑⟨p, hp''⟩ = rankAtStalk M ↑⟨x, hx⟩

Proof state:
case intro.intro.intro.mk
R : Type uR
M : Type uM
inst✝³ : CommRing R
inst✝² : AddCommGroup M
inst✝¹ : Module R M
inst✝ : FinitePresentation R M
x✝ : ↑(freeLocus R M)
x : PrimeSpectrum R
hx : x ∈ freeLocus R M
this : Free (Localization.AtPrime x.asIdeal) (LocalizedModule x.asIdeal.primeCompl M)
f : R
hf : f ∈ x.asIdeal.primeCompl
hf' : Free (Localization (Submonoid.powers f)) (LocalizedModule (Submonoid.powers f) M)
hf'' :
  finrank (Localization (Submonoid.powers f)) (LocalizedModule (Submonoid.powers f) M) =
    finrank (Localization.AtPrime x.asIdeal) (LocalizedModule x.asIdeal.primeCompl M)
p : PrimeSpectrum R
hp'' : p ∈ freeLocus R M
hp : ⟨p, hp''⟩ ∈ Subtype.val ⁻¹' ↑(basicOpen f)
⊢ rankAtStalk M ↑⟨p, hp''⟩ = rankAtStalk M ↑⟨x, hx⟩

Tactic:
let p' := Algebra.algebraMapSubmonoid (Localization (.powers f)) p.asIdeal.primeCompl

Result:
case intro.intro.intro.mk
R : Type uR
M : Type uM
inst✝³ : CommRing R
inst✝² : AddCommGroup M
inst✝¹ : Module R M
inst✝ : FinitePresentation R M
x✝ : ↑(freeLocus R M)
x : PrimeSpectrum R
hx : x ∈ freeLocus R M
this : Free (Localization.AtPrime x.asIdeal) (LocalizedModule x.asIdeal.primeCompl M)
f : R
hf : f ∈ x.asIdeal.primeCompl
hf' : Free (Localization (Submonoid.powers f)) (LocalizedModule (Submonoid.powers f) M)
hf'' :
  finrank (Localization (Submonoid.powers f)) (LocalizedModule (Submonoid.powers f) M) =
    finrank (Localization.AtPrime x.asIdeal) (LocalizedModule x.asIdeal.primeCompl M)
p : PrimeSpectrum R
hp'' : p ∈ freeLocus R M
hp : ⟨p, hp''⟩ ∈ Subtype.val ⁻¹' ↑(basicOpen f)
p' : Submonoid (Localization (Submonoid.powers f)) :=
  Algebra.algebraMapSubmonoid (Localization (Submonoid.powers f)) p.asIdeal.primeCompl
⊢ rankAtStalk M ↑⟨p, hp''⟩ = rankAtStalk M ↑⟨x, hx⟩