/-- If 0 → M' → M → M'' → 0 is exact and M' and M'' are
finitely generated then so is M. -/
theorem fg_of_fg_map_of_fg_inf_ker {R M P : Type*} [Ring R] [AddCommGroup M] [Module R M]
    [AddCommGroup P] [Module R P] (f : M →ₗ[R] P) {s : Submodule R M}
    (hs1 : (s.map f).FG)
    (hs2 : (s ⊓ LinearMap.ker f).FG) : s.FG := by
  haveI := Classical.decEq R
  haveI := Classical.decEq M
  haveI := Classical.decEq P
  cases' hs1 with t1 ht1
  cases' hs2 with t2 ht2
  have : ∀ y ∈ t1, ∃ x ∈ s, f x = y := by
    intro y hy
    have : y ∈ s.map f := by
      rw [← ht1]
      exact subset_span hy
    rcases mem_map.1 this with ⟨x, hx1, hx2⟩
    exact ⟨x, hx1, hx2⟩
  have : ∃ g : P → M, ∀ y ∈ t1, g y ∈ s ∧ f (g y) = y := by
    choose g hg1 hg2 using this
    exists fun y => if H : y ∈ t1 then g y H else 0
    intro y H
    constructor
    · simp only [dif_pos H]
      apply hg1
    · simp only [dif_pos H]
      apply hg2
  cases' this with g hg
  clear this
  exists t1.image g ∪ t2
  rw [Finset.coe_union, span_union, Finset.coe_image]
  apply le_antisymm
  · refine' sup_le (span_le.2 <| image_subset_iff.2 _) (span_le.2 _)
    · intro y hy
      exact (hg y hy).1
    · intro x hx
      have : x ∈ span R t2 := subset_span hx
      rw [ht2] at this
      exact this.1
  intro x hx
  have : f x ∈ s.map f := by
    rw [mem_map]
    exact ⟨x, hx, rfl⟩
  rw [← ht1, ← Set.image_id (t1 : Set P), Finsupp.mem_span_image_iff_total] at this
  rcases this with ⟨l, hl1, hl2⟩
  refine'
    mem_sup.2
      ⟨(Finsupp.total M M R id).toFun ((Finsupp.lmapDomain R R g : (P →₀ R) → M →₀ R) l), _,
        x - Finsupp.total M M R id ((Finsupp.lmapDomain R R g : (P →₀ R) → M →₀ R) l), _,
        add_sub_cancel'_right _ _⟩
  · rw [← Set.image_id (g '' ↑t1), Finsupp.mem_span_image_iff_total]
    refine' ⟨_, _, rfl⟩
    haveI : Inhabited P := ⟨0⟩
    rw [← Finsupp.lmapDomain_supported _ _ g, mem_map]
    refine' ⟨l, hl1, _⟩
    rfl
  rw [ht2, mem_inf]
  constructor
  · apply s.sub_mem hx
    rw [Finsupp.total_apply, Finsupp.lmapDomain_apply, Finsupp.sum_mapDomain_index]
    refine' s.sum_mem _
    · intro y hy
      exact s.smul_mem _ (hg y (hl1 hy)).1
    · exact zero_smul _
    · exact fun _ _ _ => add_smul _ _ _
  · rw [LinearMap.mem_ker, f.map_sub, ← hl2]
    rw [Finsupp.total_apply, Finsupp.total_apply, Finsupp.lmapDomain_apply]
    rw [Finsupp.sum_mapDomain_index, Finsupp.sum, Finsupp.sum, map_sum]
    rw [sub_eq_zero]
    refine' Finset.sum_congr rfl fun y hy => _
    unfold id
    

Proof state:
case intro.intro.intro.a.intro.intro.refine'_2.right
R✝ : Type u_1
M✝ : Type u_2
inst✝⁹ : Semiring R✝
inst✝⁸ : AddCommMonoid M✝
inst✝⁷ : Module R✝ M✝
P✝ : Type u_3
inst✝⁶ : AddCommMonoid P✝
inst✝⁵ : Module R✝ P✝
f✝ : M✝ →ₗ[R✝] P✝
R : Type u_4
M : Type u_5
P : Type u_6
inst✝⁴ : Ring R
inst✝³ : AddCommGroup M
inst✝² : Module R M
inst✝¹ : AddCommGroup P
inst✝ : Module R P
f : M →ₗ[R] P
s : Submodule R M
this✝¹ : DecidableEq R
this✝ : DecidableEq M
this : DecidableEq P
t1 : Finset P
ht1 : span R ↑t1 = map f s
t2 : Finset M
ht2 : span R ↑t2 = s ⊓ LinearMap.ker f
g : P → M
hg : ∀ y ∈ t1, g y ∈ s ∧ f (g y) = y
x : M
hx : x ∈ s
l : P →₀ R
hl1 : l ∈ Finsupp.supported R R ↑t1
hl2 : (Finsupp.total P P R id) l = f x
y : P
hy : y ∈ l.support
⊢ l y • y = f (l y • g y)
case intro.intro.intro.a.intro.intro.refine'_2.right.h_zero
R✝ : Type u_1
M✝ : Type u_2
inst✝⁹ : Semiring R✝
inst✝⁸ : AddCommMonoid M✝
inst✝⁷ : Module R✝ M✝
P✝ : Type u_3
inst✝⁶ : AddCommMonoid P✝
inst✝⁵ : Module R✝ P✝
f✝ : M✝ →ₗ[R✝] P✝
R : Type u_4
M : Type u_5
P : Type u_6
inst✝⁴ : Ring R
inst✝³ : AddCommGroup M
inst✝² : Module R M
inst✝¹ : AddCommGroup P
inst✝ : Module R P
f : M →ₗ[R] P
s : Submodule R M
this✝¹ : DecidableEq R
this✝ : DecidableEq M
this : DecidableEq P
t1 : Finset P
ht1 : span R ↑t1 = map f s
t2 : Finset M
ht2 : span R ↑t2 = s ⊓ LinearMap.ker f
g : P → M
hg : ∀ y ∈ t1, g y ∈ s ∧ f (g y) = y
x : M
hx : x ∈ s
l : P →₀ R
hl1 : l ∈ Finsupp.supported R R ↑t1
hl2 : (Finsupp.total P P R id) l = f x
⊢ ∀ (b : M), 0 • id b = 0
case intro.intro.intro.a.intro.intro.refine'_2.right.h_add
R✝ : Type u_1
M✝ : Type u_2
inst✝⁹ : Semiring R✝
inst✝⁸ : AddCommMonoid M✝
inst✝⁷ : Module R✝ M✝
P✝ : Type u_3
inst✝⁶ : AddCommMonoid P✝
inst✝⁵ : Module R✝ P✝
f✝ : M✝ →ₗ[R✝] P✝
R : Type u_4
M : Type u_5
P : Type u_6
inst✝⁴ : Ring R
inst✝³ : AddCommGroup M
inst✝² : Module R M
inst✝¹ : AddCommGroup P
inst✝ : Module R P
f : M →ₗ[R] P
s : Submodule R M
this✝¹ : DecidableEq R
this✝ : DecidableEq M
this : DecidableEq P
t1 : Finset P
ht1 : span R ↑t1 = map f s
t2 : Finset M
ht2 : span R ↑t2 = s ⊓ LinearMap.ker f
g : P → M
hg : ∀ y ∈ t1, g y ∈ s ∧ f (g y) = y
x : M
hx : x ∈ s
l : P →₀ R
hl1 : l ∈ Finsupp.supported R R ↑t1
hl2 : (Finsupp.total P P R id) l = f x
⊢ ∀ (b : M) (m₁ m₂ : R), (m₁ + m₂) • id b = m₁ • id b + m₂ • id b

Next tactic:
rw [f.map_smul, (hg y (hl1 hy)).2]