obtain ⟨I, hI⟩ := (M.matroidOnUniv hM).exists_basis X (by simp [hM])
α : Type u_1
I B X✝ : Set α
M : ClMatroid α
hM : M.E = univ
X : Set α
⊢ (M.matroidOnUniv hM).cl X = M.cl X
case intro
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
⊢ (M.matroidOnUniv hM).cl X = M.cl X