-- Mathlib/RingTheory/DedekindDomain/PID.lean
theorem FractionalIdeal.isPrincipal.of_finite_maximals_of_inv {A : Type*} [CommRing A]
    [Algebra R A] {S : Submonoid R} [IsLocalization S A] (hS : S ≤ R⁰)
    (hf : {I : Ideal R | I.IsMaximal}.Finite) (I I' : FractionalIdeal S A) (hinv : I * I' = 1) :
    Submodule.IsPrincipal (I : Submodule R A)

Proof state:
case intro.intro.intro.intro.intro
R : Type u_1
inst✝³ : CommRing R
A : Type u_2
inst✝² : CommRing A
inst✝¹ : Algebra R A
S : Submonoid R
inst✝ : IsLocalization S A
hS : S ≤ R⁰
hf : {I | I.IsMaximal}.Finite
I I' : FractionalIdeal S A
hinv : ↑I * ↑I' = ↑1
hinv' : I * I' = 1
s : Finset (Ideal R) := hf.toFinset
this✝ : DecidableEq (Ideal R)
coprime : ∀ M ∈ s, ∀ M' ∈ s.erase M, M ⊔ M' = ⊤
nle : ∀ M ∈ s, ¬⨅ M' ∈ s.erase M, M' ≤ M
a : Ideal R → A
ha : ∀ M ∈ s, a M ∈ I
b : Ideal R → A
hb : ∀ M ∈ s, b M ∈ I'
hm : ∀ M ∈ s, a M * b M ∉ IsLocalization.coeSubmodule A M
u : Ideal R → R
hu : ∀ M ∈ s, u M ∈ ⨅ M' ∈ s.erase M, M'
hum : ∀ M ∈ s, u M ∉ M
v : A := ∑ M ∈ s, u M • b M
hv : v ∈ I'
h : ¬Submodule.comap (Algebra.linearMap R A) (↑↑(Units.mkOfMulEqOne I I' hinv') * Submodule.span R {v}) = ⊤
M : Ideal R
hJM : Submodule.comap (Algebra.linearMap R A) (↑↑(Units.mkOfMulEqOne I I' hinv') * Submodule.span R {v}) ≤ M
hM : M ∈ hf.toFinset
this : ∀ a ∈ I, ∀ b ∈ I', ∃ c, (algebraMap R A) c = a * b
c : R
hc : (algebraMap R A) c = a M * b M
d : R
hdM : d ∈ ↑M
he : (Algebra.linearMap R A) d = (algebraMap R A) (u M * c)
⊢ (algebraMap R A) c ∈ IsLocalization.coeSubmodule A M

Tactic:
rw [IsLocalization.injective _ hS he] at hdM

Result:
case intro.intro.intro.intro.intro
R : Type u_1
inst✝³ : CommRing R
A : Type u_2
inst✝² : CommRing A
inst✝¹ : Algebra R A
S : Submonoid R
inst✝ : IsLocalization S A
hS : S ≤ R⁰
hf : {I | I.IsMaximal}.Finite
I I' : FractionalIdeal S A
hinv : ↑I * ↑I' = ↑1
hinv' : I * I' = 1
s : Finset (Ideal R) := hf.toFinset
this✝ : DecidableEq (Ideal R)
coprime : ∀ M ∈ s, ∀ M' ∈ s.erase M, M ⊔ M' = ⊤
nle : ∀ M ∈ s, ¬⨅ M' ∈ s.erase M, M' ≤ M
a : Ideal R → A
ha : ∀ M ∈ s, a M ∈ I
b : Ideal R → A
hb : ∀ M ∈ s, b M ∈ I'
hm : ∀ M ∈ s, a M * b M ∉ IsLocalization.coeSubmodule A M
u : Ideal R → R
hu : ∀ M ∈ s, u M ∈ ⨅ M' ∈ s.erase M, M'
hum : ∀ M ∈ s, u M ∉ M
v : A := ∑ M ∈ s, u M • b M
hv : v ∈ I'
h : ¬Submodule.comap (Algebra.linearMap R A) (↑↑(Units.mkOfMulEqOne I I' hinv') * Submodule.span R {v}) = ⊤
M : Ideal R
hJM : Submodule.comap (Algebra.linearMap R A) (↑↑(Units.mkOfMulEqOne I I' hinv') * Submodule.span R {v}) ≤ M
hM : M ∈ hf.toFinset
this : ∀ a ∈ I, ∀ b ∈ I', ∃ c, (algebraMap R A) c = a * b
c : R
hc : (algebraMap R A) c = a M * b M
d : R
hdM : u M * c ∈ ↑M
he : (Algebra.linearMap R A) d = (algebraMap R A) (u M * c)
⊢ (algebraMap R A) c ∈ IsLocalization.coeSubmodule A M

Proof state:
case intro.intro
R : Type u_1
inst✝³ : CommRing R
A : Type u_2
inst✝² : CommRing A
inst✝¹ : Algebra R A
S : Submonoid R
inst✝ : IsLocalization S A
hS : S ≤ R⁰
hf : {I | I.IsMaximal}.Finite
I I' : FractionalIdeal S A
hinv : ↑I * ↑I' = ↑1
hinv' : I * I' = 1
s : Finset (Ideal R) := hf.toFinset
this✝ : DecidableEq (Ideal R)
coprime : ∀ M ∈ s, ∀ M' ∈ s.erase M, M ⊔ M' = ⊤
nle : ∀ M ∈ s, ¬⨅ M' ∈ s.erase M, M' ≤ M
a : Ideal R → A
ha : ∀ M ∈ s, a M ∈ I
b : Ideal R → A
hb : ∀ M ∈ s, b M ∈ I'
hm : ∀ M ∈ s, a M * b M ∉ IsLocalization.coeSubmodule A M
u : Ideal R → R
hu : ∀ M ∈ s, u M ∈ ⨅ M' ∈ s.erase M, M'
hum : ∀ M ∈ s, u M ∉ M
v : A := ∑ M ∈ s, u M • b M
hv : v ∈ I'
h : ¬Submodule.comap (Algebra.linearMap R A) (↑↑(Units.mkOfMulEqOne I I' hinv') * Submodule.span R {v}) = ⊤
M : Ideal R
hJM : Submodule.comap (Algebra.linearMap R A) (↑↑(Units.mkOfMulEqOne I I' hinv') * Submodule.span R {v}) ≤ M
hM : M ∈ hf.toFinset
this : ∀ a ∈ I, ∀ b ∈ I', ∃ c, (algebraMap R A) c = a * b
hmem : u M • (a M * b M) ∈ IsLocalization.coeSubmodule A M
⊢ a M * b M ∈ IsLocalization.coeSubmodule A M

Tactic:
obtain ⟨c, hc : algebraMap R A c = a M * b M⟩ := this _ (ha M hM) _ (hb M hM)

Result:
case intro.intro.intro
R : Type u_1
inst✝³ : CommRing R
A : Type u_2
inst✝² : CommRing A
inst✝¹ : Algebra R A
S : Submonoid R
inst✝ : IsLocalization S A
hS : S ≤ R⁰
hf : {I | I.IsMaximal}.Finite
I I' : FractionalIdeal S A
hinv : ↑I * ↑I' = ↑1
hinv' : I * I' = 1
s : Finset (Ideal R) := hf.toFinset
this✝ : DecidableEq (Ideal R)
coprime : ∀ M ∈ s, ∀ M' ∈ s.erase M, M ⊔ M' = ⊤
nle : ∀ M ∈ s, ¬⨅ M' ∈ s.erase M, M' ≤ M
a : Ideal R → A
ha : ∀ M ∈ s, a M ∈ I
b : Ideal R → A
hb : ∀ M ∈ s, b M ∈ I'
hm : ∀ M ∈ s, a M * b M ∉ IsLocalization.coeSubmodule A M
u : Ideal R → R
hu : ∀ M ∈ s, u M ∈ ⨅ M' ∈ s.erase M, M'
hum : ∀ M ∈ s, u M ∉ M
v : A := ∑ M ∈ s, u M • b M
hv : v ∈ I'
h : ¬Submodule.comap (Algebra.linearMap R A) (↑↑(Units.mkOfMulEqOne I I' hinv') * Submodule.span R {v}) = ⊤
M : Ideal R
hJM : Submodule.comap (Algebra.linearMap R A) (↑↑(Units.mkOfMulEqOne I I' hinv') * Submodule.span R {v}) ≤ M
hM : M ∈ hf.toFinset
this : ∀ a ∈ I, ∀ b ∈ I', ∃ c, (algebraMap R A) c = a * b
hmem : u M • (a M * b M) ∈ IsLocalization.coeSubmodule A M
c : R
hc : (algebraMap R A) c = a M * b M
⊢ a M * b M ∈ IsLocalization.coeSubmodule A M

Proof state:
case intro
R : Type u_1
inst✝³ : CommRing R
A : Type u_2
inst✝² : CommRing A
inst✝¹ : Algebra R A
S : Submonoid R
inst✝ : IsLocalization S A
hS : S ≤ R⁰
hf : {I | I.IsMaximal}.Finite
I I' : FractionalIdeal S A
hinv : ↑I * ↑I' = ↑1
hinv' : I * I' = 1
s : Finset (Ideal R) := hf.toFinset
this✝ : DecidableEq (Ideal R)
coprime : ∀ M ∈ s, ∀ M' ∈ s.erase M, M ⊔ M' = ⊤
nle : ∀ M ∈ s, ¬⨅ M' ∈ s.erase M, M' ≤ M
a : Ideal R → A
ha : ∀ M ∈ s, a M ∈ I
b : Ideal R → A
hb : ∀ M ∈ s, b M ∈ I'
hm : ∀ M ∈ s, a M * b M ∉ IsLocalization.coeSubmodule A M
u : Ideal R → R
hu : ∀ M ∈ s, u M ∈ ⨅ M' ∈ s.erase M, M'
hum : ∀ M ∈ s, u M ∉ M
v : A := ∑ M ∈ s, u M • b M
hv : v ∈ I'
h : ¬Submodule.comap (Algebra.linearMap R A) (↑↑(Units.mkOfMulEqOne I I' hinv') * Submodule.span R {v}) = ⊤
M : Ideal R
hJM : Submodule.comap (Algebra.linearMap R A) (↑↑(Units.mkOfMulEqOne I I' hinv') * Submodule.span R {v}) ≤ M
hM : M ∈ hf.toFinset
this : ∀ a ∈ I, ∀ b ∈ I', ∃ c, (algebraMap R A) c = a * b
c : R
hc : (algebraMap R A) c = a M * v
⊢ c ∈ ↑(Submodule.comap (Algebra.linearMap R A) (↑↑(Units.mkOfMulEqOne I I' hinv') * Submodule.span R {v}))

Tactic:
have := Submodule.mul_mem_mul (ha M hM) (Submodule.mem_span_singleton_self v)

Result:
case intro
R : Type u_1
inst✝³ : CommRing R
A : Type u_2
inst✝² : CommRing A
inst✝¹ : Algebra R A
S : Submonoid R
inst✝ : IsLocalization S A
hS : S ≤ R⁰
hf : {I | I.IsMaximal}.Finite
I I' : FractionalIdeal S A
hinv : ↑I * ↑I' = ↑1
hinv' : I * I' = 1
s : Finset (Ideal R) := hf.toFinset
this✝¹ : DecidableEq (Ideal R)
coprime : ∀ M ∈ s, ∀ M' ∈ s.erase M, M ⊔ M' = ⊤
nle : ∀ M ∈ s, ¬⨅ M' ∈ s.erase M, M' ≤ M
a : Ideal R → A
ha : ∀ M ∈ s, a M ∈ I
b : Ideal R → A
hb : ∀ M ∈ s, b M ∈ I'
hm : ∀ M ∈ s, a M * b M ∉ IsLocalization.coeSubmodule A M
u : Ideal R → R
hu : ∀ M ∈ s, u M ∈ ⨅ M' ∈ s.erase M, M'
hum : ∀ M ∈ s, u M ∉ M
v : A := ∑ M ∈ s, u M • b M
hv : v ∈ I'
h : ¬Submodule.comap (Algebra.linearMap R A) (↑↑(Units.mkOfMulEqOne I I' hinv') * Submodule.span R {v}) = ⊤
M : Ideal R
hJM : Submodule.comap (Algebra.linearMap R A) (↑↑(Units.mkOfMulEqOne I I' hinv') * Submodule.span R {v}) ≤ M
hM : M ∈ hf.toFinset
this✝ : ∀ a ∈ I, ∀ b ∈ I', ∃ c, (algebraMap R A) c = a * b
c : R
hc : (algebraMap R A) c = a M * v
this : a M * v ∈ ↑I * Submodule.span R {v}
⊢ c ∈ ↑(Submodule.comap (Algebra.linearMap R A) (↑↑(Units.mkOfMulEqOne I I' hinv') * Submodule.span R {v}))