refine (M.cl_subset_cl_of_subset_cl
  (iUnion_subset (fun i ↦ M.cl_subset_cl (subset_iUnion _ _)))).antisymm ?_
α : Type u_1
ι : Type u_2
M✝ : Matroid α
F I J X Y B C R : Set α
e f x y : α
M : Matroid α
Xs : ι → Set α
⊢ M.cl (⋃ i, M.cl (Xs i)) = M.cl (⋃ i, Xs i)
α : Type u_1
ι : Type u_2
M✝ : Matroid α
F I J X Y B C R : Set α
e f x y : α
M : Matroid α
Xs : ι → Set α
⊢ M.cl (⋃ i, Xs i) ⊆ M.cl (⋃ i, M.cl (Xs i))