-- Mathlib/RingTheory/Localization/LocalizationLocalization.lean
theorem localization_localization_surj [IsLocalization N T] (x : T) :
    ∃ y : R × localizationLocalizationSubmodule M N,
        x * algebraMap R T y.2 = algebraMap R T y.1

Proof state:
case intro.mk.intro.mk.intro.mk
R : Type u_1
inst✝⁸ : CommSemiring R
M : Submonoid R
S : Type u_2
inst✝⁷ : CommSemiring S
inst✝⁶ : Algebra R S
N : Submonoid S
T : Type u_3
inst✝⁵ : CommSemiring T
inst✝⁴ : Algebra R T
inst✝³ : Algebra S T
inst✝² : IsScalarTower R S T
inst✝¹ : IsLocalization M S
inst✝ : IsLocalization N T
x : T
y : S
s : ↥N
eq₁ : x * (algebraMap S T) ↑(y, s).2 = (algebraMap S T) (y, s).1
z : R
t : ↥M
eq₂ : y * (algebraMap R S) ↑(z, t).2 = (algebraMap R S) (z, t).1
z' : R
t' : ↥M
eq₃ : ↑s * (algebraMap R S) ↑(z', t').2 = (algebraMap R S) (z', t').1
⊢ ∃ y, x * (algebraMap R T) ↑y.2 = (algebraMap R T) y.1

Tactic:
dsimp only at eq₁ eq₂ eq₃

Result:
case intro.mk.intro.mk.intro.mk
R : Type u_1
inst✝⁸ : CommSemiring R
M : Submonoid R
S : Type u_2
inst✝⁷ : CommSemiring S
inst✝⁶ : Algebra R S
N : Submonoid S
T : Type u_3
inst✝⁵ : CommSemiring T
inst✝⁴ : Algebra R T
inst✝³ : Algebra S T
inst✝² : IsScalarTower R S T
inst✝¹ : IsLocalization M S
inst✝ : IsLocalization N T
x : T
y : S
s : ↥N
eq₁ : x * (algebraMap S T) ↑s = (algebraMap S T) y
z : R
t : ↥M
eq₂ : y * (algebraMap R S) ↑t = (algebraMap R S) z
z' : R
t' : ↥M
eq₃ : ↑s * (algebraMap R S) ↑t' = (algebraMap R S) z'
⊢ ∃ y, x * (algebraMap R T) ↑y.2 = (algebraMap R T) y.1

Proof state:
case intro.mk.intro.mk.intro.mk.refine_2
R : Type u_1
inst✝⁸ : CommSemiring R
M : Submonoid R
S : Type u_2
inst✝⁷ : CommSemiring S
inst✝⁶ : Algebra R S
N : Submonoid S
T : Type u_3
inst✝⁵ : CommSemiring T
inst✝⁴ : Algebra R T
inst✝³ : Algebra S T
inst✝² : IsScalarTower R S T
inst✝¹ : IsLocalization M S
inst✝ : IsLocalization N T
x : T
y : S
s : ↥N
eq₁ : x * (algebraMap S T) ↑s = (algebraMap S T) y
z : R
t : ↥M
eq₂ : y * (algebraMap R S) ↑t = (algebraMap R S) z
z' : R
t' : ↥M
eq₃ : ↑s * (algebraMap R S) ↑t' = (algebraMap R S) z'
⊢ x * (algebraMap R T) ↑(z * ↑t', ⟨z' * ↑t, ⋯⟩).2 = (algebraMap R T) (z * ↑t', ⟨z' * ↑t, ⋯⟩).1

Tactic:
simp only [Subtype.coe_mk, RingHom.map_mul, IsScalarTower.algebraMap_apply R S T, ← eq₃, ← eq₂,
  ← eq₁]

Result:
case intro.mk.intro.mk.intro.mk.refine_2
R : Type u_1
inst✝⁸ : CommSemiring R
M : Submonoid R
S : Type u_2
inst✝⁷ : CommSemiring S
inst✝⁶ : Algebra R S
N : Submonoid S
T : Type u_3
inst✝⁵ : CommSemiring T
inst✝⁴ : Algebra R T
inst✝³ : Algebra S T
inst✝² : IsScalarTower R S T
inst✝¹ : IsLocalization M S
inst✝ : IsLocalization N T
x : T
y : S
s : ↥N
eq₁ : x * (algebraMap S T) ↑s = (algebraMap S T) y
z : R
t : ↥M
eq₂ : y * (algebraMap R S) ↑t = (algebraMap R S) z
z' : R
t' : ↥M
eq₃ : ↑s * (algebraMap R S) ↑t' = (algebraMap R S) z'
⊢ x * ((algebraMap S T) ↑s * (algebraMap S T) ((algebraMap R S) ↑t') * (algebraMap S T) ((algebraMap R S) ↑t)) =
    x * (algebraMap S T) ↑s * (algebraMap S T) ((algebraMap R S) ↑t) * (algebraMap S T) ((algebraMap R S) ↑t')

Proof state:
case intro.mk.intro.mk.intro.mk.refine_1
R : Type u_1
inst✝⁸ : CommSemiring R
M : Submonoid R
S : Type u_2
inst✝⁷ : CommSemiring S
inst✝⁶ : Algebra R S
N : Submonoid S
T : Type u_3
inst✝⁵ : CommSemiring T
inst✝⁴ : Algebra R T
inst✝³ : Algebra S T
inst✝² : IsScalarTower R S T
inst✝¹ : IsLocalization M S
inst✝ : IsLocalization N T
x : T
y : S
s : ↥N
eq₁ : x * (algebraMap S T) ↑s = (algebraMap S T) y
z : R
t : ↥M
eq₂ : y * (algebraMap R S) ↑t = (algebraMap R S) z
z' : R
t' : ↥M
eq₃ : ↑s * (algebraMap R S) ↑t' = (algebraMap R S) z'
⊢ (algebraMap R S) (z' * ↑t) = ↑s * (algebraMap R S) ↑(t * t')

Tactic:
rw [RingHom.map_mul, ← eq₃, mul_assoc, ← RingHom.map_mul, mul_comm t, Submonoid.coe_mul]

Result:
no goals

Proof state:
case intro.mk.intro.mk.intro.mk
R : Type u_1
inst✝⁸ : CommSemiring R
M : Submonoid R
S : Type u_2
inst✝⁷ : CommSemiring S
inst✝⁶ : Algebra R S
N : Submonoid S
T : Type u_3
inst✝⁵ : CommSemiring T
inst✝⁴ : Algebra R T
inst✝³ : Algebra S T
inst✝² : IsScalarTower R S T
inst✝¹ : IsLocalization M S
inst✝ : IsLocalization N T
x : T
y : S
s : ↥N
eq₁ : x * (algebraMap S T) ↑s = (algebraMap S T) y
z : R
t : ↥M
eq₂ : y * (algebraMap R S) ↑t = (algebraMap R S) z
z' : R
t' : ↥M
eq₃ : ↑s * (algebraMap R S) ↑t' = (algebraMap R S) z'
⊢ ∃ y, x * (algebraMap R T) ↑y.2 = (algebraMap R T) y.1

Tactic:
refine ⟨⟨z * t', z' * t, ?_⟩, ?_⟩

Result:
case intro.mk.intro.mk.intro.mk.refine_1
R : Type u_1
inst✝⁸ : CommSemiring R
M : Submonoid R
S : Type u_2
inst✝⁷ : CommSemiring S
inst✝⁶ : Algebra R S
N : Submonoid S
T : Type u_3
inst✝⁵ : CommSemiring T
inst✝⁴ : Algebra R T
inst✝³ : Algebra S T
inst✝² : IsScalarTower R S T
inst✝¹ : IsLocalization M S
inst✝ : IsLocalization N T
x : T
y : S
s : ↥N
eq₁ : x * (algebraMap S T) ↑s = (algebraMap S T) y
z : R
t : ↥M
eq₂ : y * (algebraMap R S) ↑t = (algebraMap R S) z
z' : R
t' : ↥M
eq₃ : ↑s * (algebraMap R S) ↑t' = (algebraMap R S) z'
⊢ z' * ↑t ∈ localizationLocalizationSubmodule M N

case intro.mk.intro.mk.intro.mk.refine_2
R : Type u_1
inst✝⁸ : CommSemiring R
M : Submonoid R
S : Type u_2
inst✝⁷ : CommSemiring S
inst✝⁶ : Algebra R S
N : Submonoid S
T : Type u_3
inst✝⁵ : CommSemiring T
inst✝⁴ : Algebra R T
inst✝³ : Algebra S T
inst✝² : IsScalarTower R S T
inst✝¹ : IsLocalization M S
inst✝ : IsLocalization N T
x : T
y : S
s : ↥N
eq₁ : x * (algebraMap S T) ↑s = (algebraMap S T) y
z : R
t : ↥M
eq₂ : y * (algebraMap R S) ↑t = (algebraMap R S) z
z' : R
t' : ↥M
eq₃ : ↑s * (algebraMap R S) ↑t' = (algebraMap R S) z'
⊢ x * (algebraMap R T) ↑(z * ↑t', ⟨z' * ↑t, ?intro.mk.intro.mk.intro.mk.refine_1⟩).2 =
    (algebraMap R T) (z * ↑t', ⟨z' * ↑t, ?intro.mk.intro.mk.intro.mk.refine_1⟩).1

Proof state:
case intro.mk.intro.mk.intro.mk.refine_1
R : Type u_1
inst✝⁸ : CommSemiring R
M : Submonoid R
S : Type u_2
inst✝⁷ : CommSemiring S
inst✝⁶ : Algebra R S
N : Submonoid S
T : Type u_3
inst✝⁵ : CommSemiring T
inst✝⁴ : Algebra R T
inst✝³ : Algebra S T
inst✝² : IsScalarTower R S T
inst✝¹ : IsLocalization M S
inst✝ : IsLocalization N T
x : T
y : S
s : ↥N
eq₁ : x * (algebraMap S T) ↑s = (algebraMap S T) y
z : R
t : ↥M
eq₂ : y * (algebraMap R S) ↑t = (algebraMap R S) z
z' : R
t' : ↥M
eq₃ : ↑s * (algebraMap R S) ↑t' = (algebraMap R S) z'
⊢ ∃ y z, (algebraMap R S) (z' * ↑t) = ↑y * (algebraMap R S) ↑z

Tactic:
refine ⟨s, t * t', ?_⟩

Result:
case intro.mk.intro.mk.intro.mk.refine_1
R : Type u_1
inst✝⁸ : CommSemiring R
M : Submonoid R
S : Type u_2
inst✝⁷ : CommSemiring S
inst✝⁶ : Algebra R S
N : Submonoid S
T : Type u_3
inst✝⁵ : CommSemiring T
inst✝⁴ : Algebra R T
inst✝³ : Algebra S T
inst✝² : IsScalarTower R S T
inst✝¹ : IsLocalization M S
inst✝ : IsLocalization N T
x : T
y : S
s : ↥N
eq₁ : x * (algebraMap S T) ↑s = (algebraMap S T) y
z : R
t : ↥M
eq₂ : y * (algebraMap R S) ↑t = (algebraMap R S) z
z' : R
t' : ↥M
eq₃ : ↑s * (algebraMap R S) ↑t' = (algebraMap R S) z'
⊢ (algebraMap R S) (z' * ↑t) = ↑s * (algebraMap R S) ↑(t * t')

Proof state:
case intro.mk
R : Type u_1
inst✝⁸ : CommSemiring R
M : Submonoid R
S : Type u_2
inst✝⁷ : CommSemiring S
inst✝⁶ : Algebra R S
N : Submonoid S
T : Type u_3
inst✝⁵ : CommSemiring T
inst✝⁴ : Algebra R T
inst✝³ : Algebra S T
inst✝² : IsScalarTower R S T
inst✝¹ : IsLocalization M S
inst✝ : IsLocalization N T
x : T
y : S
s : ↥N
eq₁ : x * (algebraMap S T) ↑(y, s).2 = (algebraMap S T) (y, s).1
⊢ ∃ y, x * (algebraMap R T) ↑y.2 = (algebraMap R T) y.1

Tactic:
rcases IsLocalization.surj M y with ⟨⟨z, t⟩, eq₂⟩

Result:
case intro.mk.intro.mk
R : Type u_1
inst✝⁸ : CommSemiring R
M : Submonoid R
S : Type u_2
inst✝⁷ : CommSemiring S
inst✝⁶ : Algebra R S
N : Submonoid S
T : Type u_3
inst✝⁵ : CommSemiring T
inst✝⁴ : Algebra R T
inst✝³ : Algebra S T
inst✝² : IsScalarTower R S T
inst✝¹ : IsLocalization M S
inst✝ : IsLocalization N T
x : T
y : S
s : ↥N
eq₁ : x * (algebraMap S T) ↑(y, s).2 = (algebraMap S T) (y, s).1
z : R
t : ↥M
eq₂ : y * (algebraMap R S) ↑(z, t).2 = (algebraMap R S) (z, t).1
⊢ ∃ y, x * (algebraMap R T) ↑y.2 = (algebraMap R T) y.1