simp_rw [← hI.cl_eq_cl, subset_def, hI.indep.mem_cl_iff']
case intro.refine_2
α : Type u_1
I✝ B X✝ : Set α
M : ClMatroid α
hM : M.E = univ
X I : Set α
hI : (M.matroidOnUniv hM).Basis I X
hi : (M.indepMatroidOnUniv hM).Indep I
⊢ M.cl X ⊆ (M.matroidOnUniv hM).cl X
case intro.refine_2
α : Type u_1
I✝ B X✝ : Set α
M : ClMatroid α
hM : M.E = univ
X I : Set α
hI : (M.matroidOnUniv hM).Basis I X
hi : (M.indepMatroidOnUniv hM).Indep I
⊢ ∀ x ∈ M.cl X, x ∈ (M.matroidOnUniv hM).E ∧ ((M.matroidOnUniv hM).Indep (insert x I) → x ∈ I)