-- Lean 4 proof step from Mathlib.NumberTheory.NumberField.CanonicalEmbedding.Basic (goal)
Goal:
K : Type u_1
inst : Field K
inst_1 : NumberField K
x : K
i_1 : Free.ChooseBasisIndex ℤ (𝓞 K)
f : K →ₗ[ℚ]
  ↥(Submodule.span ℚ
      (Set.range (ι := Free.ChooseBasisIndex ℤ (𝓞 K))
        (⇑(latticeBasis K : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℂ ((K →+* ℂ) → ℂ)) :
          (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → (K →+* ℂ) → ℂ))) :=
  LinearMap.codRestrict
    (Submodule.span ℚ
      (Set.range (ι := Free.ChooseBasisIndex ℤ (𝓞 K))
        (⇑(latticeBasis K : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℂ ((K →+* ℂ) → ℂ)) :
          (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → (K →+* ℂ) → ℂ)))
    (AlgHom.toLinearMap (RingHom.toRatAlgHom (canonicalEmbedding K))) ⋯
i : Free.ChooseBasisIndex ℤ (𝓞 K)
this :
  Eq (α :=
    ↥(Submodule.span ℚ
        (Set.range (ι := Free.ChooseBasisIndex ℤ (𝓞 K))
          (⇑(latticeBasis K : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℂ ((K →+* ℂ) → ℂ)) :
            (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → (K →+* ℂ) → ℂ))))
    ((f :
          (a : K) →
            ↥(Submodule.span ℚ (Set.range (⇑(latticeBasis K) : (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → (K →+* ℂ) → ℂ))))
        (((integralBasis K : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℚ K) : (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → K) i) :
      ↥(Submodule.span ℚ
          (Set.range (ι := Free.ChooseBasisIndex ℤ (𝓞 K))
            (⇑(latticeBasis K : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℂ ((K →+* ℂ) → ℂ)) :
              (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → (K →+* ℂ) → ℂ))))
    (((Basis.restrictScalars ℚ (latticeBasis K) :
            Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℚ
              ↥(Submodule.span ℚ
                  (Set.range (ι := Free.ChooseBasisIndex ℤ (𝓞 K))
                    (⇑(latticeBasis K : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℂ ((K →+* ℂ) → ℂ)) :
                      (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → (K →+* ℂ) → ℂ)))) :
          (a : Free.ChooseBasisIndex ℤ (𝓞 K)) →
            ↥(Submodule.span ℚ (Set.range (⇑(latticeBasis K) : (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → (K →+* ℂ) → ℂ))))
        i :
      ↥(Submodule.span ℚ
          (Set.range (ι := Free.ChooseBasisIndex ℤ (𝓞 K))
            (⇑(latticeBasis K : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℂ ((K →+* ℂ) → ℂ)) :
              (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → (K →+* ℂ) → ℂ))))
⊢ Eq (α := Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ)
    ((((↑(Basis.repr (Basis.restrictScalars ℚ (latticeBasis K))) :
                ↥(Submodule.span ℚ
                      (Set.range (⇑(latticeBasis K) : (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → (K →+* ℂ) → ℂ))) →ₗ[ℚ]
                  Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ) ∘ₗ
              f :
            K →ₗ[ℚ] Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ) :
          (a : K) → Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ)
        (((integralBasis K : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℚ K) : (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → K) i :
          K) :
      Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ)
    ((((↑(Basis.repr (integralBasis K)) : K →ₗ[ℚ] Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ) :
            K →ₗ[ℚ] Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ) :
          (a : K) → Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ)
        (((integralBasis K : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℚ K) : (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → K) i :
          K) :
      Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ)
have simp_rw : Eq (α := Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ)
  (Function.comp (α := K) (β :=
      ↥(Submodule.span ℚ
          (Set.range (ι := Free.ChooseBasisIndex ℤ (𝓞 K))
            (⇑(latticeBasis K : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℂ ((K →+* ℂ) → ℂ)) :
              (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → (K →+* ℂ) → ℂ))))
      (⇑((↑(Basis.repr (Basis.restrictScalars ℚ (latticeBasis K))) :
              ↥(Submodule.span ℚ
                    (Set.range (ι := Free.ChooseBasisIndex ℤ (𝓞 K))
                      (⇑(latticeBasis K : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℂ ((K →+* ℂ) → ℂ)) :
                        (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → (K →+* ℂ) → ℂ))) →ₗ[ℚ]
                Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ) :
            ↥(Submodule.span ℚ
                  (Set.range (ι := Free.ChooseBasisIndex ℤ (𝓞 K))
                    (⇑(latticeBasis K : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℂ ((K →+* ℂ) → ℂ)) :
                      (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → (K →+* ℂ) → ℂ))) →ₗ[ℚ]
              Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ) :
        (a :
            ↥(Submodule.span ℚ (Set.range (⇑(latticeBasis K) : (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → (K →+* ℂ) → ℂ)))) →
          Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ)
      ((⇑f :
          (a : K) →
            ↥(Submodule.span ℚ
                (Set.range (ι := Free.ChooseBasisIndex ℤ (𝓞 K))
                  (⇑(latticeBasis K : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℂ ((K →+* ℂ) → ℂ)) :
                    (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → (K →+* ℂ) → ℂ)))) :
        (a : K) →
          ↥(Submodule.span ℚ
              (Set.range (ι := Free.ChooseBasisIndex ℤ (𝓞 K))
                (⇑(latticeBasis K : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℂ ((K →+* ℂ) → ℂ)) :
                  (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → (K →+* ℂ) → ℂ))))
      (((integralBasis K : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℚ K) : (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → K) i :
        K) :
    Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ)
  ((((↑(Basis.repr (integralBasis K)) : K →ₗ[ℚ] Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ) :
          K →ₗ[ℚ] Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ) :
        (a : K) → Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ)
      (((integralBasis K : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℚ K) : (a : Free.ChooseBasisIndex ℤ (𝓞 K)) → K) i :
        K) :
    Free.ChooseBasisIndex ℤ (𝓞 K) →₀ ℚ)
Tactic: simp_rw [LinearMap.coe_comp, LinearEquiv.coe_coe, Function.comp_apply, this, Basis.repr_self]