-- Lean 4 proof step from Mathlib.NumberTheory.NumberField.CanonicalEmbedding.FundamentalCone (goal)
Goal:
K : Type u_1
inst : Field K
inst_1 : NumberField K
b : Associates ↥(𝓞 K)⁰
x : ↥(𝓞 K)⁰
u : (𝓞 K)ˣ
hu :
  Membership.mem (γ := Set (mixedSpace K)) (integerSet K)
    (HSMul.hSMul (β := mixedSpace K) u ((mixedEmbedding K : (a : K) → mixedSpace K) (↑(↑x : 𝓞 K) : K) : mixedSpace K))
mk :
  (↑(preimageOfMemIntegerSet
            (Subtype.mk (p := fun (x : mixedSpace K) ↦ Membership.mem (γ := Set (mixedSpace K)) (integerSet K) x)
              (HSMul.hSMul (β := mixedSpace K) u
                ((mixedEmbedding K : (a : K) → mixedSpace K) (↑(↑x : 𝓞 K) : K) : mixedSpace K))
              hu) *
          (↑((MulEquiv.symm unitsNonZeroDivisorsEquiv : (a : (𝓞 K)ˣ) → (↥(𝓞 K)⁰)ˣ) u⁻¹) : ↥(𝓞 K)⁰)) :
      𝓞 K) =
    (↑x : 𝓞 K)
mk₁ :
  (↑(↑(preimageOfMemIntegerSet
                (Subtype.mk (p := fun (x : mixedSpace K) ↦ Membership.mem (γ := Set (mixedSpace K)) (integerSet K) x)
                  (HSMul.hSMul (β := mixedSpace K) u
                    ((mixedEmbedding K : (a : K) → mixedSpace K) (↑(↑x : 𝓞 K) : K) : mixedSpace K))
                  hu) *
              (↑((MulEquiv.symm unitsNonZeroDivisorsEquiv : (a : (𝓞 K)ˣ) → (↥(𝓞 K)⁰)ˣ) u⁻¹) : ↥(𝓞 K)⁰)) :
          𝓞 K) :
      K) =
    (↑(↑x : 𝓞 K) : K)
mk₂ :
  Eq (α := mixedSpace K)
    ((mixedEmbedding K : (a : K) → mixedSpace K)
        (↑(↑(preimageOfMemIntegerSet
                    (Subtype.mk (p := fun (x : mixedSpace K) ↦
                      Membership.mem (γ := Set (mixedSpace K)) (integerSet K) x)
                      (HSMul.hSMul (β := mixedSpace K) u
                        ((mixedEmbedding K : (a : K) → mixedSpace K) (↑(↑x : 𝓞 K) : K) : mixedSpace K))
                      hu) *
                  (↑((MulEquiv.symm unitsNonZeroDivisorsEquiv : (a : (𝓞 K)ˣ) → (↥(𝓞 K)⁰)ˣ) u⁻¹) : ↥(𝓞 K)⁰)) :
              𝓞 K) :
          K) :
      mixedSpace K)
    ((mixedEmbedding K : (a : K) → mixedSpace K) (↑(↑x : 𝓞 K) : K) : mixedSpace K)
⊢ preimageOfMemIntegerSet
        (Subtype.mk (p := fun (x : mixedSpace K) ↦ Membership.mem (γ := Set (mixedSpace K)) (integerSet K) x)
          (HSMul.hSMul (β := mixedSpace K) u
            ((mixedEmbedding K : (a : K) → mixedSpace K) (↑(↑x : 𝓞 K) : K) : mixedSpace K))
          hu) *
      (↑((MulEquiv.symm unitsNonZeroDivisorsEquiv : (a : (𝓞 K)ˣ) → (↥(𝓞 K)⁰)ˣ) u⁻¹) : ↥(𝓞 K)⁰) =
    x
have mk₃ : Eq (α := mixedSpace K)
  ((mixedEmbedding K : (a : K) → mixedSpace K)
      (↑((↑(preimageOfMemIntegerSet
                  (Subtype.mk (p := fun (x : mixedSpace K) ↦ Membership.mem (γ := Set (mixedSpace K)) (integerSet K) x)
                    (HSMul.hSMul (β := mixedSpace K) u
                      ((mixedEmbedding K : (a : K) → mixedSpace K) (↑(↑x : 𝓞 K) : K) : mixedSpace K))
                    hu)) :
              𝓞 K) *
            (↑(↑((MulEquiv.symm unitsNonZeroDivisorsEquiv : (a : (𝓞 K)ˣ) → (↥(𝓞 K)⁰)ˣ) u⁻¹) : ↥(𝓞 K)⁰) : 𝓞 K)) :
        K) :
    mixedSpace K)
  ((mixedEmbedding K : (a : K) → mixedSpace K) (↑(↑x : 𝓞 K) : K) : mixedSpace K)
Tactic: simp_rw [Subtype.ext_iff, RingOfIntegers.ext_iff, ← (mixedEmbedding_injective K).eq_iff,
    Submonoid.coe_mul, map_mul, mixedEmbedding_preimageOfMemIntegerSet,
    unitSMul_smul, ← map_mul, mul_comm, map_inv, val_inv_unitsNonZeroDivisorsEquiv_symm_apply_coe,
    Units.mul_inv_cancel_right]