exact M.er_mono (hI.subset.trans inter_subset_left)
case intro
α : Type u_1
M✝ N : Matroid α
I✝ J C✝ D X✝ Y Z : Set α
e f : α
M : Matroid α
C X I : Set α
hI : (M ／ C).Basis I (X ∩ (M ／ C).E)
⊢ M.er I ≤ M.er X
no goals