rw [← er_inter_ground_eq, erk_eq_er_ground, union_diff_self,
union_inter_cancel_right]
α : Type u_1
ι : Type u_2
M✝ N : Matroid α
I B X✝ X' Y Y' Z R : Set α
n : ℕ∞
e f : α
M : Matroid α
X : Set α
⊢ M.erk ≤ M.er (X ∪ M.E \ X)
no goals