Theorem: Matroid.delete_er_eq_delete_er_diff (Matroid/Minor/Rank.lean)

State before:
α : Type u_1
M✝ N : Matroid α
I J C D✝ X✝ Y Z : Set α
e f : α
M : Matroid α
D X : Set α
⊢ (M ＼ D).er X = (M ＼ D).er (X \ D)
Tactic:
simp
State after:
no goals