obtain ⟨B, hB, hIB⟩ := h.exists_base_superset
ι : Type u_1
α : Type u_2
M : Matroid α
B I J X X' Y Y' F : Set α
Xs Ys Js Is : ι → Set α
i j : ι
e f : α
h : M.Indep (⋃ i, Xs i)
⊢ ∃ B, M.ModularBase B Xs
case intro.intro
ι : Type u_1
α : Type u_2
M : Matroid α
B✝ I J X X' Y Y' F : Set α
Xs Ys Js Is : ι → Set α
i j : ι
e f : α
h : M.Indep (⋃ i, Xs i)
B : Set α
hB : M.Base B
hIB : ⋃ i, Xs i ⊆ B
⊢ ∃ B, M.ModularBase B Xs