rwa [insert_diff_self_of_not_mem xnI, hI.cl_eq_cl]
case intro.refine'_2.inl
α : Type u_1
M : Matroid α
I✝ B : Set α
Ys Xs : Set (Set α)
X : Set α
C : M.ModularCut
hX : M.cl X ∈ C
I : Set α
hI : M.Basis' I X
x : α
he : x ∉ M.E
ex_basis : (C.extension x).Basis' I X
xnI : x ∉ I
hf : x ∈ insert x I \ M.E ∧ M.Indep (insert x I \ {x}) ∧ M.cl (insert x I \ {x}) ∉ C
⊢ M.cl (insert x I \ {x}) ∈ C
no goals