refine' M.cl_subset_cl (iUnion_subset (fun i ↦ (M.subset_cl _).trans _))
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.E) ⊆ M.cl (⋃ i, M.cl (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
i : ι
⊢ M.cl (Xs i ∩ M.E) ⊆ ⋃ i, M.cl (Xs i)