exact ⟨hB.indep.basis_cl, hB.subset.trans (union_subset_union hI.subset hJ.subset)⟩
α : Type u_1
M✝ : Matroid α
B✝ B' I✝ I' J✝ K X✝ Y✝ : Set α
M : Matroid α
X Y I : Set α
hI : M.Basis' I X
J : Set α
hJ : M.Basis' J Y
B : Set α
hB : M.Basis' B (I ∪ J)
⊢ M.Basis B (M.cl B) ∧ B ⊆ X ∪ Y
no goals