/-- The valuation ring `A` is isomorphic to the ring of integers of its associated valuation. -/
noncomputable def equivInteger : A ≃+* (valuation A K).integer :=
  RingEquiv.ofBijective
    (show A →ₙ+* (valuation A K).integer from
      { toFun := fun a => ⟨algebraMap A K a, (mem_integer_iff _ _ _).mpr ⟨a, rfl⟩⟩
        map_mul' := fun _ _ => by ext1; exact (algebraMap A K).map_mul _ _
        map_zero' := by ext1; exact (algebraMap A K).map_zero
        map_add' := fun _ _ => by ext1; exact (algebraMap A K).map_add _ _ })
    (by
      constructor
      · intro x y h
        

Proof state:
case left
A : Type u
inst✝⁵ : CommRing A
K : Type v
inst✝⁴ : Field K
inst✝³ : Algebra A K
inst✝² : IsDomain A
inst✝¹ : ValuationRing A
inst✝ : IsFractionRing A K
x y : A
h :
  (let_fun this :=
        {
          toMulHom :=
            {
              toFun := fun a =>
                { val := (algebraMap A K) a, property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) },
              map_mul' :=
                (_ :
                  ∀ (x x_1 : A),
                    (fun a =>
                          { val := (algebraMap A K) a,
                            property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                        (x * x_1) =
                      (fun a =>
                            { val := (algebraMap A K) a,
                              property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                          x *
                        (fun a =>
                            { val := (algebraMap A K) a,
                              property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                          x_1) },
          map_zero' :=
            (_ :
              MulHom.toFun
                  {
                    toFun := fun a =>
                      { val := (algebraMap A K) a,
                        property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) },
                    map_mul' :=
                      (_ :
                        ∀ (x x_1 : A),
                          (fun a =>
                                { val := (algebraMap A K) a,
                                  property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                              (x * x_1) =
                            (fun a =>
                                  { val := (algebraMap A K) a,
                                    property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                x *
                              (fun a =>
                                  { val := (algebraMap A K) a,
                                    property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                x_1) }
                  0 =
                0),
          map_add' :=
            (_ :
              ∀ (x x_1 : A),
                MulHom.toFun
                    {
                      toFun := fun a =>
                        { val := (algebraMap A K) a,
                          property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) },
                      map_mul' :=
                        (_ :
                          ∀ (x x_2 : A),
                            (fun a =>
                                  { val := (algebraMap A K) a,
                                    property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                (x * x_2) =
                              (fun a =>
                                    { val := (algebraMap A K) a,
                                      property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                  x *
                                (fun a =>
                                    { val := (algebraMap A K) a,
                                      property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                  x_2) }
                    (x + x_1) =
                  MulHom.toFun
                      {
                        toFun := fun a =>
                          { val := (algebraMap A K) a,
                            property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) },
                        map_mul' :=
                          (_ :
                            ∀ (x x_2 : A),
                              (fun a =>
                                    { val := (algebraMap A K) a,
                                      property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                  (x * x_2) =
                                (fun a =>
                                      { val := (algebraMap A K) a,
                                        property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                    x *
                                  (fun a =>
                                      { val := (algebraMap A K) a,
                                        property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                    x_2) }
                      x +
                    MulHom.toFun
                      {
                        toFun := fun a =>
                          { val := (algebraMap A K) a,
                            property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) },
                        map_mul' :=
                          (_ :
                            ∀ (x x_2 : A),
                              (fun a =>
                                    { val := (algebraMap A K) a,
                                      property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                  (x * x_2) =
                                (fun a =>
                                      { val := (algebraMap A K) a,
                                        property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                    x *
                                  (fun a =>
                                      { val := (algebraMap A K) a,
                                        property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                    x_2) }
                      x_1) };
      this)
      x =
    (let_fun this :=
        {
          toMulHom :=
            {
              toFun := fun a =>
                { val := (algebraMap A K) a, property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) },
              map_mul' :=
                (_ :
                  ∀ (x x_1 : A),
                    (fun a =>
                          { val := (algebraMap A K) a,
                            property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                        (x * x_1) =
                      (fun a =>
                            { val := (algebraMap A K) a,
                              property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                          x *
                        (fun a =>
                            { val := (algebraMap A K) a,
                              property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                          x_1) },
          map_zero' :=
            (_ :
              MulHom.toFun
                  {
                    toFun := fun a =>
                      { val := (algebraMap A K) a,
                        property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) },
                    map_mul' :=
                      (_ :
                        ∀ (x x_1 : A),
                          (fun a =>
                                { val := (algebraMap A K) a,
                                  property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                              (x * x_1) =
                            (fun a =>
                                  { val := (algebraMap A K) a,
                                    property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                x *
                              (fun a =>
                                  { val := (algebraMap A K) a,
                                    property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                x_1) }
                  0 =
                0),
          map_add' :=
            (_ :
              ∀ (x x_1 : A),
                MulHom.toFun
                    {
                      toFun := fun a =>
                        { val := (algebraMap A K) a,
                          property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) },
                      map_mul' :=
                        (_ :
                          ∀ (x x_2 : A),
                            (fun a =>
                                  { val := (algebraMap A K) a,
                                    property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                (x * x_2) =
                              (fun a =>
                                    { val := (algebraMap A K) a,
                                      property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                  x *
                                (fun a =>
                                    { val := (algebraMap A K) a,
                                      property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                  x_2) }
                    (x + x_1) =
                  MulHom.toFun
                      {
                        toFun := fun a =>
                          { val := (algebraMap A K) a,
                            property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) },
                        map_mul' :=
                          (_ :
                            ∀ (x x_2 : A),
                              (fun a =>
                                    { val := (algebraMap A K) a,
                                      property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                  (x * x_2) =
                                (fun a =>
                                      { val := (algebraMap A K) a,
                                        property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                    x *
                                  (fun a =>
                                      { val := (algebraMap A K) a,
                                        property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                    x_2) }
                      x +
                    MulHom.toFun
                      {
                        toFun := fun a =>
                          { val := (algebraMap A K) a,
                            property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) },
                        map_mul' :=
                          (_ :
                            ∀ (x x_2 : A),
                              (fun a =>
                                    { val := (algebraMap A K) a,
                                      property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                  (x * x_2) =
                                (fun a =>
                                      { val := (algebraMap A K) a,
                                        property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                    x *
                                  (fun a =>
                                      { val := (algebraMap A K) a,
                                        property := (_ : (algebraMap A K) a ∈ Valuation.integer (valuation A K)) })
                                    x_2) }
                      x_1) };
      this)
      y
⊢ x = y

Next tactic:
apply_fun (algebraMap (valuation A K).integer K) at h