-- Lean 4 proof step from Mathlib.LinearAlgebra.RootSystem.GeckConstruction.Basic (goal)
Goal:
ι : Type u_1
R : Type u_2
M : Type u_3
N : Type u_4
inst : CommRing R
inst_1 : AddCommGroup M
inst_2 : Module R M
inst_3 : AddCommGroup N
inst_4 : Module R N
P : RootPairing ι R M N
inst_5 : P.IsCrystallographic
b : P.Base
inst_6 : Finite ι
inst_7 : IsDomain R
inst_8 : CharZero R
inst_9 : DecidableEq ι
inst_10 : Fintype ι
x : Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R
hx : Membership.mem (γ := LieSubalgebra R (Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)) (lieAlgebra b) x
mem :
  ∀ (u : Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)
    (hu :
      Membership.mem (γ := Set (Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R))
        (range h ∪ range e ∪ range f) u),
    Membership.mem (α := Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) (γ :=
      LieSubalgebra R (Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)) (lieAlgebra b)
      ((ωConj b : (a : Matrix (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) → Matrix (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) u :
        Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)
zero :
  Membership.mem (γ := LieSubalgebra R (Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)) (lieAlgebra b)
    ((ωConj b : (a : Matrix (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) → Matrix (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) 0)
add :
  ∀ (u v : Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R),
    Membership.mem (γ := LieSubalgebra R (Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R))
        (LieSubalgebra.lieSpan R (Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)
          (range h ∪ range e ∪ range f))
        u →
      Membership.mem (γ := LieSubalgebra R (Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R))
          (LieSubalgebra.lieSpan R (Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)
            (range h ∪ range e ∪ range f))
          v →
        ∀
          (hu :
            Membership.mem (α := Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) (γ :=
              LieSubalgebra R (Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)) (lieAlgebra b)
              ((ωConj b : (a : Matrix (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) → Matrix (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)
                  u :
                Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R))
          (hv :
            Membership.mem (α := Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) (γ :=
              LieSubalgebra R (Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)) (lieAlgebra b)
              ((ωConj b : (a : Matrix (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) → Matrix (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)
                  v :
                Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)),
          Membership.mem (α := Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) (γ :=
            LieSubalgebra R (Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)) (lieAlgebra b)
            ((ωConj b : (a : Matrix (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) → Matrix (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)
                (u + v) :
              Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)
⊢ Membership.mem (α := Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) (γ :=
    LieSubalgebra R (Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)) (lieAlgebra b)
    ((ωConj b : (a : Matrix (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) → Matrix (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) x :
      Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)
have smul : ∀ (t : R) (u : Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R),
  Membership.mem (γ := LieSubalgebra R (Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R))
      (LieSubalgebra.lieSpan R (Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)
        (range h ∪ range e ∪ range f))
      u →
    ∀
      (hu :
        Membership.mem (α := Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) (γ :=
          LieSubalgebra R (Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)) (lieAlgebra b)
          ((ωConj b : (a : Matrix (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) → Matrix (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) u :
            Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)),
      Membership.mem (α := Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) (γ :=
        LieSubalgebra R (Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)) (lieAlgebra b)
        ((ωConj b : (a : Matrix (↥b.support ⊕ ι) (↥b.support ⊕ ι) R) → Matrix (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)
            (t • u) :
          Matrix.{u_1, u_1, u_2} (↥b.support ⊕ ι) (↥b.support ⊕ ι) R)
Tactic: induction hx using LieSubalgebra.lieSpan_induction with
  | mem u hu =>
    obtain (⟨i, rfl⟩ | ⟨i, rfl⟩ | ⟨i, rfl⟩) : (∃ j, h j = u) ∨ (∃ j, e j = u) ∨ (∃ j, f j = u) := by
      simpa only [mem_union, mem_range, or_assoc] using hu
    · rw [← neg_mem_iff]
      exact LieSubalgebra.subset_lieSpan <| by simp [ω_mul_h, mul_assoc]
    · exact LieSubalgebra.subset_lieSpan <| by simp [ω_mul_e, mul_assoc]
    · exact LieSubalgebra.subset_lieSpan <| by simp [ω_mul_f, mul_assoc]
  | zero => simp
  | add u v _ _ hu hv => simpa [mul_add, add_mul] using add_mem hu hv
  | smul t u _ hu => simpa using SMulMemClass.smul_mem _ hu
  | lie u v _ _ hu hv =>
    rw [LieEquiv.map_lie]
    exact (lieAlgebra b).lie_mem hu hv
Cleared hypotheses: hx