-- Mathlib/RingTheory/RingHom/Locally.lean
lemma locally_iff_finite (f : R →+* S) :
    Locally P f ↔ ∃ (s : Finset S) (_ : Ideal.span (s : Set S) = ⊤),
      ∀ t ∈ s, P ((algebraMap S (Localization.Away t)).comp f)

Proof state:
case mpr
P : {R S : Type u} → [inst : CommRing R] → [inst_1 : CommRing S] → (R →+* S) → Prop
R S : Type u
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Finset S
hsone : Ideal.span ↑s = ⊤
hs : ∀ t ∈ s, P ((algebraMap S (Localization.Away t)).comp f)
⊢ Locally (fun {R S} [CommRing R] [CommRing S] => P) f

Tactic:
use s, hsone, hs

Result:
no goals

Proof state:
case mp
P : {R S : Type u} → [inst : CommRing R] → [inst_1 : CommRing S] → (R →+* S) → Prop
R S : Type u
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Set S
hsone : Ideal.span s = ⊤
hs : ∀ t ∈ s, (fun {R S} [CommRing R] [CommRing S] => P) ((algebraMap S (Localization.Away t)).comp f)
⊢ ∃ s, ∃ (_ : Ideal.span ↑s = ⊤), ∀ t ∈ s, P ((algebraMap S (Localization.Away t)).comp f)

Tactic:
obtain ⟨s', h₁, h₂⟩ := (Ideal.span_eq_top_iff_finite s).mp hsone

Result:
case mp.intro.intro
P : {R S : Type u} → [inst : CommRing R] → [inst_1 : CommRing S] → (R →+* S) → Prop
R S : Type u
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Set S
hsone : Ideal.span s = ⊤
hs : ∀ t ∈ s, (fun {R S} [CommRing R] [CommRing S] => P) ((algebraMap S (Localization.Away t)).comp f)
s' : Finset S
h₁ : ↑s' ⊆ s
h₂ : Ideal.span ↑s' = ⊤
⊢ ∃ s, ∃ (_ : Ideal.span ↑s = ⊤), ∀ t ∈ s, P ((algebraMap S (Localization.Away t)).comp f)

Proof state:
case mp.intro.intro
P : {R S : Type u} → [inst : CommRing R] → [inst_1 : CommRing S] → (R →+* S) → Prop
R S : Type u
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Set S
hsone : Ideal.span s = ⊤
hs : ∀ t ∈ s, (fun {R S} [CommRing R] [CommRing S] => P) ((algebraMap S (Localization.Away t)).comp f)
s' : Finset S
h₁ : ↑s' ⊆ s
h₂ : Ideal.span ↑s' = ⊤
⊢ ∃ s, ∃ (_ : Ideal.span ↑s = ⊤), ∀ t ∈ s, P ((algebraMap S (Localization.Away t)).comp f)

Tactic:
exact ⟨s', h₂, fun t ht ↦ hs t (h₁ ht)⟩

Result:
no goals

Proof state:
case mpr
P : {R S : Type u} → [inst : CommRing R] → [inst_1 : CommRing S] → (R →+* S) → Prop
R S : Type u
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
⊢ (∃ s, ∃ (_ : Ideal.span ↑s = ⊤), ∀ t ∈ s, P ((algebraMap S (Localization.Away t)).comp f)) →
    Locally (fun {R S} [CommRing R] [CommRing S] => P) f

Tactic:
intro ⟨s, hsone, hs⟩

Result:
case mpr
P : {R S : Type u} → [inst : CommRing R] → [inst_1 : CommRing S] → (R →+* S) → Prop
R S : Type u
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Finset S
hsone : Ideal.span ↑s = ⊤
hs : ∀ t ∈ s, P ((algebraMap S (Localization.Away t)).comp f)
⊢ Locally (fun {R S} [CommRing R] [CommRing S] => P) f

Proof state:
case mp
P : {R S : Type u} → [inst : CommRing R] → [inst_1 : CommRing S] → (R →+* S) → Prop
R S : Type u
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
⊢ Locally (fun {R S} [CommRing R] [CommRing S] => P) f →
    ∃ s, ∃ (_ : Ideal.span ↑s = ⊤), ∀ t ∈ s, P ((algebraMap S (Localization.Away t)).comp f)

Tactic:
intro ⟨s, hsone, hs⟩

Result:
case mp
P : {R S : Type u} → [inst : CommRing R] → [inst_1 : CommRing S] → (R →+* S) → Prop
R S : Type u
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Set S
hsone : Ideal.span s = ⊤
hs : ∀ t ∈ s, (fun {R S} [CommRing R] [CommRing S] => P) ((algebraMap S (Localization.Away t)).comp f)
⊢ ∃ s, ∃ (_ : Ideal.span ↑s = ⊤), ∀ t ∈ s, P ((algebraMap S (Localization.Away t)).comp f)

Proof state:
P : {R S : Type u} → [inst : CommRing R] → [inst_1 : CommRing S] → (R →+* S) → Prop
R S : Type u
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
⊢ Locally (fun {R S} [CommRing R] [CommRing S] => P) f ↔
    ∃ s, ∃ (_ : Ideal.span ↑s = ⊤), ∀ t ∈ s, P ((algebraMap S (Localization.Away t)).comp f)

Tactic:
constructor

Result:
case mp
P : {R S : Type u} → [inst : CommRing R] → [inst_1 : CommRing S] → (R →+* S) → Prop
R S : Type u
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
⊢ Locally (fun {R S} [CommRing R] [CommRing S] => P) f →
    ∃ s, ∃ (_ : Ideal.span ↑s = ⊤), ∀ t ∈ s, P ((algebraMap S (Localization.Away t)).comp f)

case mpr
P : {R S : Type u} → [inst : CommRing R] → [inst_1 : CommRing S] → (R →+* S) → Prop
R S : Type u
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
⊢ (∃ s, ∃ (_ : Ideal.span ↑s = ⊤), ∀ t ∈ s, P ((algebraMap S (Localization.Away t)).comp f)) →
    Locally (fun {R S} [CommRing R] [CommRing S] => P) f