/-- If `b` mod `p` spans `S/p` as `R/p`-space, then `b` itself spans `Frac(S)` as `K`-space.

Here,
 * `p` is an ideal of `R` such that `R / p` is nontrivial
 * `K` is a field that has an embedding of `R` (in particular we can take `K = Frac(R)`)
 * `L` is a field extension of `K`
 * `S` is the integral closure of `R` in `L`

More precisely, we avoid quotients in this statement and instead require that `b ∪ pS` spans `S`.
-/
theorem FinrankQuotientMap.span_eq_top [IsDomain R] [IsDomain S] [Algebra K L] [IsNoetherian R S]
    [Algebra R L] [IsScalarTower R S L] [IsScalarTower R K L] [IsIntegralClosure S R L]
    [NoZeroSMulDivisors R K] (hp : p ≠ ⊤) (b : Set S)
    (hb' : Submodule.span R b ⊔ (p.map (algebraMap R S)).restrictScalars R = ⊤) :
    Submodule.span K (algebraMap S L '' b) = ⊤ := by
  have hRL : Function.Injective (algebraMap R L) := by
    rw [IsScalarTower.algebraMap_eq R K L]
    exact (algebraMap K L).injective.comp (NoZeroSMulDivisors.algebraMap_injective R K)
  -- Let `M` be the `R`-module spanned by the proposed basis elements.
  let M : Submodule R S := Submodule.span R b
  -- Then `S / M` is generated by some finite set of `n` vectors `a`.
  letI h : Module.Finite R (S ⧸ M) :=
    Module.Finite.of_surjective (Submodule.mkQ _) (Submodule.Quotient.mk_surjective _)
  obtain ⟨n, a, ha⟩ := @Module.Finite.exists_fin _ _ _ _ _ h
  -- Because the image of `p` in `S / M` is `⊤`,
  have smul_top_eq : p • (⊤ : Submodule R (S ⧸ M)) = ⊤ := by
    calc
      p • ⊤ = Submodule.map M.mkQ (p • ⊤) := by
        rw [Submodule.map_smul'', Submodule.map_top, M.range_mkQ]
      _ = ⊤ := by rw [Ideal.smul_top_eq_map, (Submodule.map_mkQ_eq_top M _).mpr hb']
  -- we can write the elements of `a` as `p`-linear combinations of other elements of `a`.
  have exists_sum : ∀ x : S ⧸ M, ∃ a' : Fin n → R, (∀ i, a' i ∈ p) ∧ ∑ i, a' i • a i = x := by
    intro x
    obtain ⟨a'', ha'', hx⟩ := (Submodule.mem_ideal_smul_span_iff_exists_sum p a x).1
      (by { rw [ha, smul_top_eq]; exact Submodule.mem_top } :
        x ∈ p • Submodule.span R (Set.range a))
    · refine ⟨fun i => a'' i, fun i => ha'' _, ?_⟩
      

Proof state:
case intro.intro
R : Type u
inst✝²⁶ : CommRing R
S : Type v
inst✝²⁵ : CommRing S
f : R →+* S
p : Ideal R
P : Ideal S
inst✝²⁴ : Algebra R S
K : Type u_1
inst✝²³ : Field K
inst✝²² : Algebra R K
hRK : IsFractionRing R K
L : Type u_2
inst✝²¹ : Field L
inst✝²⁰ : Algebra S L
inst✝¹⁹ : IsFractionRing S L
V : Type u_3
V' : Type u_4
V'' : Type u_5
inst✝¹⁸ : AddCommGroup V
inst✝¹⁷ : Module R V
inst✝¹⁶ : Module K V
inst✝¹⁵ : IsScalarTower R K V
inst✝¹⁴ : AddCommGroup V'
inst✝¹³ : Module R V'
inst✝¹² : Module S V'
inst✝¹¹ : IsScalarTower R S V'
inst✝¹⁰ : AddCommGroup V''
inst✝⁹ : Module R V''
inst✝⁸ : IsDomain R
inst✝⁷ : IsDomain S
inst✝⁶ : Algebra K L
inst✝⁵ : IsNoetherian R S
inst✝⁴ : Algebra R L
inst✝³ : IsScalarTower R S L
inst✝² : IsScalarTower R K L
inst✝¹ : IsIntegralClosure S R L
inst✝ : NoZeroSMulDivisors R K
hp : p ≠ ⊤
b : Set S
hb' : Submodule.span R b ⊔ Submodule.restrictScalars R (map (algebraMap R S) p) = ⊤
hRL : Function.Injective ⇑(algebraMap R L)
M : Submodule R S := Submodule.span R b
h : Module.Finite R (S ⧸ M) := Module.Finite.of_surjective (Submodule.mkQ M) (Submodule.Quotient.mk_surjective M)
n : ℕ
a : Fin n → S ⧸ M
ha : Submodule.span R (Set.range a) = ⊤
smul_top_eq : p • ⊤ = ⊤
x : S ⧸ M
a'' : Fin n →₀ R
ha'' : ∀ (i : Fin n), a'' i ∈ p
hx : (Finsupp.sum a'' fun i c => c • a i) = x
⊢ ∑ i : Fin n, (fun i => a'' i) i • a i = x

Next tactic:
rw [← hx, Finsupp.sum_fintype]