-- Lean 4 proof step from Mathlib.RingTheory.Invariant.Basic (goal)
Goal:
A : Type u_1
B : Type u_2
inst : CommRing A
inst_1 : CommRing B
inst_2 : Algebra A B
G : Type u_3
inst_3 : Group G
inst_4 : Finite G
inst_5 : MulSemiringAction G B
inst_6 : SMulCommClass G A B
P : Ideal A
Q : Ideal B
inst_7 : Q.IsPrime
inst_8 : Q.LiesOver P
K : Type u_4
L : Type u_5
inst_9 : Field K
inst_10 : Field L
inst_11 : Algebra.{u_1, u_4} (A ⧸ P) K
inst_12 : Algebra.{u_2, u_5} (B ⧸ Q) L
inst_13 : Algebra.{u_1, u_5} (A ⧸ P) L
inst_14 : IsScalarTower.{u_1, u_2, u_5} (A ⧸ P) (B ⧸ Q) L
inst_15 : Algebra K L
inst_16 : IsScalarTower.{u_1, u_4, u_5} (A ⧸ P) K L
inst_17 : Algebra.IsInvariant A B G
inst_18 : Module.IsTorsionFree.{u_2, u_5} (B ⧸ Q) L
f : Gal(L/K)
b : B ⧸ Q
hx :
  ∀ (g : ↥(MulAction.stabilizer G Q)),
    ((((Ideal.Quotient.stabilizerHom Q P G :
                  MonoidHom.{u_3, u_2} (↥(MulAction.stabilizer G Q))
                    (AlgEquiv.{u_1, u_2, u_2} (A ⧸ P) (B ⧸ Q) (B ⧸ Q))) :
                (a : ↥(MulAction.stabilizer G Q)) → (B ⧸ Q) ≃ₐ[A ⧸ P] B ⧸ Q)
              g :
            AlgEquiv.{u_1, u_2, u_2} (A ⧸ P) (B ⧸ Q) (B ⧸ Q)) :
          (a : B ⧸ Q) → B ⧸ Q)
        b =
      b
⊢ Eq (α := L) ((f : (a : L) → L) ((algebraMap (B ⧸ Q) L : (a : B ⧸ Q) → L) b : L) : L)
    ((algebraMap (B ⧸ Q) L : (a : B ⧸ Q) → L) b : L)
have intro : ∀ (val : Fintype G),
  Eq (α := L) ((f : (a : L) → L) ((algebraMap (B ⧸ Q) L : (a : B ⧸ Q) → L) b : L) : L)
    ((algebraMap (B ⧸ Q) L : (a : B ⧸ Q) → L) b : L)
Tactic: classical
  cases nonempty_fintype G
  obtain ⟨b₀, rfl⟩ := Ideal.Quotient.mk_surjective b
  rw [← Ideal.Quotient.algebraMap_eq]
  obtain ⟨a, b, ha1, ha2, hb⟩ := fixed_of_fixed1_aux2 G Q b₀ (fun g hg ↦ hx ⟨g, hg⟩)
  obtain ⟨M, key⟩ := (mem_lifts _).mp (Algebra.IsInvariant.charpoly_mem_lifts A B G b)
  replace key := congrArg (map (algebraMap B (B ⧸ Q))) key
  rw [map_map, ← algebraMap_eq, algebraMap_eq A (A ⧸ P) (B ⧸ Q),
      ← map_map, MulSemiringAction.charpoly, Polynomial.map_prod] at key
  have key₀ : ∀ g : G, (X - C (g • b)).map (algebraMap B (B ⧸ Q)) =
      if g • Q = Q then X - C (algebraMap B (B ⧸ Q) (a * b₀)) else X := by
    intro g
    rw [Polynomial.map_sub, map_X, map_C, hb]
    split_ifs
    · rfl
    · rw [map_zero, map_zero, sub_zero]
  simp only [key₀, Finset.prod_ite, Finset.prod_const] at key
  replace key := congrArg (map (algebraMap (B ⧸ Q) L)) key
  rw [map_map, ← algebraMap_eq, algebraMap_eq (A ⧸ P) K L,
      ← map_map, Polynomial.map_mul, Polynomial.map_pow, Polynomial.map_pow, Polynomial.map_sub,
      map_X, map_C] at key
  replace key := fixed_of_fixed1_aux3 key f (Finset.card_ne_zero_of_mem
    (Finset.mem_filter.mpr ⟨Finset.mem_univ 1, one_smul G Q⟩))
  simp only [map_mul] at key
  obtain ⟨a, rfl⟩ := Algebra.IsInvariant.isInvariant (A := A) a ha1
  rwa [← algebraMap_apply A B (B ⧸ Q), algebraMap_apply A (A ⧸ P) (B ⧸ Q),
      ← algebraMap_apply, algebraMap_apply (A ⧸ P) K L, f.commutes, mul_right_inj'] at key
  rwa [← algebraMap_apply, algebraMap_apply (A ⧸ P) (B ⧸ Q) L,
      ← algebraMap_apply A (A ⧸ P) (B ⧸ Q), algebraMap_apply A B (B ⧸ Q),
      Ne, algebraMap_eq_zero_iff, Ideal.Quotient.algebraMap_eq, Ideal.Quotient.eq_zero_iff_mem]